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P2042 Reductant Temperature Sensor Circuit Malfunction
P2042 Reductant Temperature Sensor Circuit Malfunction is a generic OBD-II diagnostic trouble code indicating that the Engine Control Module (ECM), Powertrain Control Module (PCM), or aftertreatment control module has detected a malfunction in the electrical circuit of the reductant temperature sensor.
In modern diesel vehicles, the reductant is usually Diesel Exhaust Fluid (DEF) or AdBlue, which is used by the Selective Catalytic Reduction (SCR) system to reduce nitrogen oxide (NOx) emissions.
The reductant temperature sensor monitors the temperature of the DEF/AdBlue and sends a signal to the vehicle's control system. This information can be used to control:
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DEF/AdBlue heating
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Reductant pumping
-
Reductant dosing
-
Freeze protection
-
SCR operation
-
Aftertreatment diagnostics
P2042 is a relatively broad circuit malfunction code. Unlike P2044 (circuit low), P2045 (circuit high), and P2043 (range/performance), P2042 does not identify a specific signal condition. It indicates that the control module has detected that the temperature sensor circuit is not operating correctly.
The problem may be caused by the sensor itself, wiring, connector, power supply, ground, reference circuit, reductant tank module, or control module.
What Does P2042 Mean?
The code description can be divided into several parts.
Reductant
Reductant is the fluid used in the SCR system to reduce NOx emissions.
Depending on the vehicle and market, it may be called:
-
Diesel Exhaust Fluid (DEF)
-
AdBlue
-
AUS 32
-
Urea solution
The fluid is injected into the exhaust system upstream of the SCR catalyst.
Temperature Sensor
The reductant temperature sensor measures the temperature of the DEF/AdBlue or the temperature within the reductant tank/module.
The control module uses this information to determine the thermal condition of the reductant system.
This information may affect:
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Heater operation
-
Freeze protection
-
Reductant dosing
-
Pump operation
-
SCR monitoring
-
System diagnostics
Circuit
The temperature sensor circuit can include:
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Reductant temperature sensor
-
Sensor signal wire
-
Reference-voltage circuit
-
Sensor ground
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Electrical connector
-
Wiring harness
-
Reductant tank module
-
Aftertreatment control module
-
ECM/PCM
The exact configuration varies by manufacturer.
Malfunction
"Malfunction" means the control module has identified a problem with the sensor circuit but the fault does not necessarily fit the more specific low, high, range/performance, or intermittent descriptions.
Possible electrical faults include:
-
Open circuit
-
Short circuit
-
Incorrect resistance
-
Missing reference voltage
-
Poor ground
-
Damaged connector
-
Corroded terminals
-
Wiring damage
-
Internal sensor failure
How Does the Reductant Temperature Sensor Work?
Many reductant temperature sensors use an NTC thermistor, or Negative Temperature Coefficient thermistor.
An NTC thermistor generally has:
-
Higher resistance at lower temperature
-
Lower resistance at higher temperature
The control module applies a reference voltage to the sensor circuit and uses the resulting electrical signal to calculate temperature.
In simplified terms:
Temperature decreases → resistance increases
Temperature increases → resistance decreases
The control module expects the signal to remain within a predictable electrical and physical range.
If the circuit is open, shorted, disconnected, or otherwise malfunctioning, the control module can no longer reliably determine the reductant temperature and may store P2042.
The exact sensor design and electrical values are manufacturer-specific.
Why Does the Vehicle Need a Reductant Temperature Sensor?
DEF/AdBlue behaves differently depending on temperature.
One of the most important characteristics is that DEF freezes at approximately −11°C (12°F).
The SCR system therefore needs to know when the reductant is cold enough to require heating.
The temperature information can help the control system determine:
-
When to activate the reductant heater
-
How long heating should continue
-
Whether the reductant is available for dosing
-
Whether the measured temperature is plausible
-
Whether the SCR system is operating normally
If the temperature sensor circuit fails, the control module may use substitute values or protective strategies.
Symptoms of P2042
The symptoms can vary considerably depending on the vehicle and the exact circuit failure.
Check Engine Light
The Check Engine Light is a common symptom.
The control module may illuminate the warning after detecting the reductant temperature sensor malfunction.
DEF/AdBlue Warning
The vehicle may display messages such as:
-
Check DEF
-
Check AdBlue
-
DEF system fault
-
AdBlue system fault
-
Exhaust fluid system fault
-
SCR system fault
-
Emissions system fault
The exact wording depends on the manufacturer.
Incorrect Reductant Temperature Reading
A diagnostic scan tool may show an implausible or missing reductant temperature.
For example, the temperature may:
-
Remain fixed
-
Suddenly jump
-
Show an extreme value
-
Fail to change as the system warms
-
Disagree with ambient conditions
Reductant Heater Problems
A failed temperature sensor circuit can affect heater control.
Possible symptoms include:
-
Heater does not activate correctly
-
Heater operates at the wrong time
-
Longer DEF warm-up
-
Additional heater fault codes
SCR System Warnings
The vehicle may detect additional faults involving:
-
Reductant pressure
-
Reductant dosing
-
NOx sensors
-
SCR efficiency
-
Reductant heater
-
Aftertreatment communication
Reduced Engine Power
Some diesel vehicles may enter a reduced-power strategy when persistent SCR or emissions-system faults are present.
DEF/AdBlue Countdown
Depending on the vehicle, an unresolved emissions-system fault may eventually result in:
-
DEF/AdBlue countdown
-
Speed limitation
-
Reduced engine performance
-
Restart restrictions
-
Other operating limitations
No Noticeable Driving Problems
P2042 may initially cause only a warning light.
The vehicle can sometimes continue to drive normally because the reductant temperature sensor is part of the aftertreatment system rather than the basic engine-control system.
However, the fault should not be ignored.
Common Causes of P2042
Faulty Reductant Temperature Sensor
The sensor itself may have failed.
Possible failures include:
-
Internal open circuit
-
Internal short circuit
-
Incorrect resistance
-
Damaged thermistor
-
Unstable signal
-
Incorrect temperature response
Open Circuit
A broken wire or disconnected connector can interrupt the sensor circuit.
Possible causes include:
-
Broken signal wire
-
Broken ground wire
-
Disconnected connector
-
Damaged internal tank wiring
-
Failed sensor
Short Circuit
A sensor signal wire may be shorted to ground or another circuit.
Possible causes include:
-
Chafed insulation
-
Melted wiring
-
Incorrect previous repair
-
Harness damage
-
Connector contamination
Damaged Wiring Harness
The DEF/AdBlue tank and aftertreatment components are often located in areas exposed to harsh environmental conditions.
The harness may be affected by:
-
Water
-
Road salt
-
Dirt
-
Vibration
-
Road debris
-
Exhaust heat
-
Abrasion
Corroded Connector
Corrosion can interfere with the temperature sensor circuit.
Inspect for:
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Green corrosion
-
White deposits
-
Rust
-
Moisture
-
Loose terminals
-
Bent pins
-
Damaged seals
Water Intrusion
Moisture inside a connector can cause:
-
Short circuits
-
Corrosion
-
Signal instability
-
Leakage current
-
Increased resistance
Poor Sensor Ground
A weak or high-resistance sensor ground can cause abnormal sensor operation.
A voltage-drop test can help identify a ground problem.
Reference Voltage Problem
If the sensor uses a reference-voltage circuit and the voltage is missing or incorrect, the sensor signal may become invalid.
A shared reference-voltage fault can sometimes affect other sensors.
Poor Electrical Connection
Loose or damaged terminals may cause the sensor circuit to malfunction even when a simple continuity test appears normal.
Terminal tension and connector condition should therefore be checked.
Internal Reductant Tank Wiring Fault
On some vehicles, the reductant temperature sensor is located inside the DEF/AdBlue tank module.
Internal wiring may develop:
-
Open circuits
-
Short circuits
-
High resistance
-
Poor connections
Faulty Reductant Tank Module
If the temperature sensor is integrated into the tank module, a failure of the sensor or internal electronics may require replacement of the module.
Reductant Heater Problem
A heater problem may affect the thermal behavior of the reductant system and can sometimes occur alongside temperature-sensor faults.
However, the heater should not automatically be blamed for P2042.
The temperature sensor circuit should be tested first.
DEF/AdBlue Freezing
DEF freezes at approximately −11°C (12°F).
Freezing is a normal characteristic of DEF and does not automatically mean the temperature sensor circuit has failed.
The vehicle's SCR system is designed to handle cold or frozen reductant through its heating strategy.
Incorrect or Contaminated DEF
Contaminated or incorrect DEF can cause SCR problems, but it is generally not the first suspect for a reductant temperature sensor circuit malfunction.
The electrical circuit should be tested before blaming the fluid.
Faulty Control Module
In rare cases, the ECM, PCM, or aftertreatment control module may have an internal fault affecting the temperature sensor circuit.
The control module should normally be considered only after the sensor, wiring, connector, reference voltage, and ground have been verified.
Software or Calibration Problem
Incorrect software or calibration can occasionally result in improper monitoring or interpretation of the temperature sensor circuit.
Vehicles Commonly Affected by P2042
P2042 can occur on diesel vehicles equipped with SCR and reductant temperature monitoring.
Examples may include:
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Ford F-250 Super Duty
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Ford F-350 Super Duty
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Ford F-450 Super Duty
-
Ford Transit Diesel
-
Chevrolet Silverado Duramax
-
Chevrolet Express Diesel
-
GMC Sierra Duramax
-
GMC Savana Diesel
-
Ram 2500
-
Ram 3500
-
Mercedes-Benz Sprinter
-
Volkswagen Touareg TDI
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Volkswagen Transporter
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Audi Q7 TDI
-
BMW X5 Diesel
-
Peugeot Boxer Diesel
-
Citroën Jumper
-
Fiat Ducato Diesel
-
Iveco Daily
-
Renault Master
-
Opel Movano
This list is not exhaustive.
The exact location and design of the reductant temperature sensor vary by vehicle. On some models, the sensor is integrated into the DEF/AdBlue tank module.
How Is P2042 Diagnosed?
Because P2042 is a general circuit malfunction code, diagnosis should cover the entire electrical path rather than assuming the sensor itself is defective.
Step 1: Scan for Additional Trouble Codes
Use a suitable diagnostic scan tool to retrieve:
-
Stored codes
-
Pending codes
-
Manufacturer-specific codes
-
Freeze-frame data
Look for related codes involving:
-
Reductant temperature
-
Reductant heater
-
DEF/AdBlue level
-
Reductant pressure
-
Reductant pump
-
NOx sensors
-
SCR efficiency
-
Aftertreatment communication
Additional codes can help identify whether P2042 is an isolated sensor fault or part of a larger SCR problem.
Step 2: Review Freeze-Frame Data
Check the conditions present when P2042 was stored.
Useful parameters include:
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Engine RPM
-
Vehicle speed
-
Engine coolant temperature
-
Ambient temperature
-
Reductant temperature
-
Battery voltage
-
Reductant level
-
Reductant pressure
-
Heater status
This can help determine when the circuit malfunction occurred.
Step 3: Check the Live Reductant Temperature PID
Monitor the reductant temperature using a scan tool.
Compare it with:
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Ambient temperature
-
Engine coolant temperature
-
Other available temperature sensors
-
Actual operating conditions
An impossible or missing reading is an important clue.
Step 4: Inspect the Sensor Connector
Locate the reductant temperature sensor or tank-module connector.
Check for:
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Corrosion
-
Water
-
DEF contamination
-
Loose terminals
-
Bent pins
-
Damaged seals
-
Broken locking mechanism
Step 5: Inspect the Wiring Harness
Follow the harness from the reductant tank toward the relevant control module.
Look for:
-
Broken wires
-
Chafing
-
Melted insulation
-
Pinched wires
-
Exposed conductors
-
Poor previous repairs
Pay particular attention to areas near the exhaust system.
Step 6: Check Reference Voltage
Using the manufacturer's wiring diagram, identify the sensor reference circuit and measure its voltage.
Verify that it is within specification.
If the reference voltage is missing or incorrect, investigate the circuit before replacing the sensor.
Step 7: Check Sensor Ground
Verify the sensor ground.
A voltage-drop test can reveal excessive resistance in the ground path.
Step 8: Check Sensor Signal
Measure the temperature sensor signal according to the manufacturer's procedure.
The signal should change logically with temperature.
Depending on the sensor design, abnormal results can indicate:
-
Open circuit
-
Short circuit
-
Failed sensor
-
Reference-voltage problem
-
Ground problem
-
Connector fault
Step 9: Check Sensor Resistance
If the temperature sensor is separately serviceable, disconnect it and measure its resistance.
Compare the measurement with the manufacturer's temperature-versus-resistance specifications.
Check whether:
-
Resistance is within specification
-
Resistance changes smoothly
-
Resistance is stuck
-
Resistance is excessively high
-
Resistance is excessively low
Do not use a generic resistance value because reductant temperature sensors differ between manufacturers.
Step 10: Check for an Open Circuit
Test the sensor wiring for continuity according to the manufacturer's procedure.
Check:
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Signal wire
-
Ground wire
-
Reference circuit
-
Connectors
-
Harness splices
-
Internal tank wiring where applicable
Step 11: Check for Shorts
Test for unwanted connections between:
-
Sensor signal and ground
-
Sensor signal and power
-
Sensor wires and other circuits
A short circuit can prevent the control module from correctly interpreting the temperature signal.
Step 12: Perform a Wiggle Test
Monitor the sensor signal while carefully moving the harness and connector.
If the signal changes unexpectedly, investigate:
-
Loose terminal
-
Broken conductor
-
Chafed wiring
-
Connector problem
-
Internal tank wiring
Step 13: Check for High Resistance
A circuit may show continuity while still having excessive resistance.
Inspect:
-
Connector terminals
-
Splices
-
Ground points
-
Harness sections
Voltage-drop testing can help identify high-resistance connections.
Step 14: Check Reductant Heater Operation
If the vehicle uses a reductant heater, verify its operation and look for related heater codes.
The temperature signal should behave logically as the system warms.
Step 15: Check the Reductant Tank Module
If the sensor is integrated into the DEF/AdBlue tank module, follow the manufacturer's diagnostic procedure for the module.
Do not automatically replace the tank assembly solely because P2042 is stored.
Step 16: Check Control-Module Wiring
Test the wiring between the reductant sensor/tank module and the relevant control module.
Check for:
-
Open circuit
-
Short to ground
-
Short to voltage
-
High resistance
-
Poor terminal contact
Step 17: Check Manufacturer Technical Information
Search for applicable:
-
Technical Service Bulletins
-
Wiring updates
-
Known tank-module failures
-
Connector problems
-
Software updates
-
Calibration procedures
Step 18: Check the Control Module
If the sensor and entire circuit are proven good, investigate the relevant control module.
A faulty module input or output circuit is possible, but it should be considered after external causes have been eliminated.
How to Fix P2042
The correct repair depends on the actual cause of the circuit malfunction.
Replace the Faulty Reductant Temperature Sensor
If the sensor fails resistance or signal testing and is separately serviceable, replace it with the correct component.
Repair an Open Circuit
Repair or replace broken wiring.
The harness should be correctly routed and protected against:
-
Heat
-
Abrasion
-
Vibration
-
Moisture
Repair a Short Circuit
If the sensor signal is shorted to ground, voltage, or another circuit, repair the damaged wiring and eliminate the source of the short.
Repair or Replace the Connector
Replace damaged terminals or the connector when necessary.
Make sure the repaired connector is properly sealed against moisture.
Repair the Sensor Ground
Repair:
-
Broken ground wires
-
Corroded ground points
-
Loose terminals
-
High-resistance connections
Repair the Reference-Voltage Circuit
If the sensor reference voltage is missing or incorrect because of a wiring problem, repair the affected circuit.
Repair Internal Tank Wiring
If the manufacturer provides a procedure for repairing internal reductant tank wiring, follow the specified repair procedure.
Replace the Reductant Tank Module
If the temperature sensor is integrated into the DEF/AdBlue tank module and is confirmed defective, the module may need replacement.
Repair the Reductant Heater
If testing confirms that the heater or its circuit is causing the problem, repair the heater system.
Repair or Replace the Control Module
If the sensor and wiring are proven good but the control module has a confirmed internal circuit fault, the relevant module may require repair or replacement.
Update Control Module Software
If the manufacturer has released software addressing the fault, reprogram the applicable module.
Perform Required Calibration
After replacing the sensor, tank module, or control module, the vehicle may require:
-
DEF/AdBlue system initialization
-
Reductant level reset
-
Temperature sensor calibration
-
Tank-module programming
-
SCR reset
-
Aftertreatment self-test
The exact procedure is manufacturer-specific.
Clear the Code and Verify the Repair
After completing the repair:
-
Clear P2042.
-
Start the engine.
-
Monitor the reductant temperature PID.
-
Verify sensor signal voltage.
-
Check reference voltage and ground.
-
Confirm that the temperature reading is physically plausible.
-
Check reductant heater operation if applicable.
-
Test-drive the vehicle.
-
Rescan for stored and pending codes.
The repair should be considered successful only when the temperature sensor circuit operates normally and P2042 does not return.
What Happens If P2042 Is Ignored?
If the control module cannot reliably determine reductant temperature, it may not be able to manage the SCR system correctly.
Possible consequences include:
-
DEF/AdBlue warnings
-
Incorrect reductant heating
-
Incorrect dosing
-
Reduced SCR efficiency
-
Increased NOx emissions
-
Additional aftertreatment codes
-
Reduced engine power
-
Speed restrictions
-
Restart restrictions on some vehicles
The vehicle may continue to operate normally at first, but persistent SCR faults can eventually trigger operating restrictions.
Can You Drive With P2042?
Short-term driving may be possible if the vehicle is operating normally, but P2042 should be diagnosed promptly.
If the only symptom is a Check Engine Light or DEF/AdBlue warning, the vehicle may remain driveable.
However, continued operation can lead to additional SCR warnings or restrictions depending on the vehicle.
If the vehicle displays:
-
DEF/AdBlue countdown
-
Reduced-power warning
-
Emissions-system warning
-
Speed limitation
-
Restart warning
the vehicle should be inspected as soon as possible.
Is P2042 a Serious Code?
P2042 is generally considered a moderate-severity emissions-system fault.
It does not necessarily indicate an immediate engine failure.
However, a malfunctioning reductant temperature sensor circuit can interfere with:
-
DEF/AdBlue heating
-
Reductant dosing
-
Freeze protection
-
SCR operation
-
NOx emissions control
If ignored, the fault can develop into a more significant aftertreatment problem and may eventually result in reduced engine power or operating restrictions.
P2042 vs. P2043 vs. P2044 vs. P2045 vs. P2046
These codes are closely related but describe different types of reductant temperature sensor faults.
| Code | General Meaning |
|---|---|
| P2042 | Reductant Temperature Sensor Circuit Malfunction |
| P2043 | Reductant Temperature Sensor Circuit Range / Performance |
| P2044 | Reductant Temperature Sensor Circuit Low |
| P2045 | Reductant Temperature Sensor Circuit High |
| P2046 | Reductant Temperature Sensor Circuit Intermittent |
P2042 — Circuit Malfunction
A general fault has been detected in the reductant temperature sensor circuit.
Possible causes include:
-
Open circuit
-
Short circuit
-
Sensor failure
-
Reference-voltage problem
-
Ground problem
-
Wiring or connector fault
P2043 — Range / Performance
The sensor produces a signal, but the signal or temperature behavior does not correspond to expected conditions.
Possible causes include:
-
Implausible temperature
-
Slow response
-
Stuck reading
-
Erratic response
-
Incorrect sensor resistance
P2044 — Circuit Low
The sensor signal is lower than the expected electrical range.
Possible causes include:
-
Short to ground
-
Low reference voltage
-
Sensor internal short
-
Wiring problem
P2045 — Circuit High
The sensor signal is higher than the expected electrical range.
Possible causes include:
-
Open circuit
-
Disconnected sensor
-
Broken signal wire
-
Missing ground
-
High sensor resistance
P2046 — Circuit Intermittent
The sensor signal becomes unstable or temporarily disappears.
Possible causes include:
-
Loose connector
-
Broken conductor
-
Chafed wiring
-
Corroded terminal
-
Intermittent sensor fault
A simple way to remember the sequence is:
-
P2042 → General circuit malfunction
-
P2043 → Range / performance
-
P2044 → Low
-
P2045 → High
-
P2046 → Intermittent
P2042 vs. P2044 and P2045: Why the Distinction Matters
The three codes can point toward similar components but represent different diagnostic conditions.
P2042 is a broad circuit malfunction. The control module knows that the temperature sensor circuit is not operating correctly but does not necessarily identify a specific high or low signal.
P2044 specifically indicates a low signal.
P2045 specifically indicates a high signal.
Therefore, when P2042 is stored, the technician should not immediately assume the sensor itself is defective.
For example, a disconnected connector, broken wire, missing reference voltage, poor ground, or internal tank wiring problem could all produce a general circuit malfunction.
P2042 vs. P2047, P2048 and P2049
The next group of codes concerns the reductant injection valve rather than the temperature sensor.
| Code | General Meaning |
|---|---|
| P2042 | Reductant Temperature Sensor Circuit Malfunction |
| P2047 | Reductant Injection Valve Circuit/Open Bank 1 Unit 1 |
| P2048 | Reductant Injection Valve Circuit Low Bank 1 Unit 1 |
| P2049 | Reductant Injection Valve Circuit High Bank 1 Unit 1 |
The distinction is important.
P2042 concerns the reductant temperature sensor circuit.
P2047 concerns an open reductant injection-valve circuit.
P2048 concerns a low reductant injection-valve circuit.
P2049 concerns a high reductant injection-valve circuit.
Therefore, P2042 does not automatically indicate a defective DEF/AdBlue injector. The temperature sensor circuit should be diagnosed separately.
How to Prevent P2042
Not every electrical failure can be prevented, but regular maintenance can reduce the risk.
Recommended practices include:
-
Inspect DEF/AdBlue wiring during routine servicing.
-
Repair damaged harnesses promptly.
-
Keep reductant-system connectors properly sealed.
-
Protect wiring from exhaust heat and abrasion.
-
Inspect wiring around the DEF tank.
-
Maintain the vehicle battery and charging system.
-
Use the correct DEF/AdBlue specification.
-
Avoid contaminating the reductant system.
-
Address DEF and emissions warnings promptly.
-
Inspect wiring after exhaust or tank repairs.
-
Follow manufacturer procedures after replacing reductant components.
Final Thoughts
P2042 Reductant Temperature Sensor Circuit Malfunction indicates that the vehicle's control system has detected a fault in the electrical circuit used to monitor reductant temperature.
The possible causes include:
-
Faulty reductant temperature sensor
-
Open circuit
-
Short circuit
-
Damaged wiring
-
Corroded connector
-
Water intrusion
-
Poor sensor ground
-
Reference-voltage problem
-
High-resistance connection
-
Internal reductant tank wiring fault
-
Faulty reductant tank module
-
Reductant heater-related problems
-
Control-module faults
-
Software or calibration issues
The key difference between P2042 and the following codes is that P2042 is a general circuit malfunction, while P2043 describes a range/performance problem, P2044 identifies a low signal, P2045 identifies a high signal, and P2046 identifies an intermittent signal.
For this reason, P2042 should be diagnosed by examining the complete sensor circuit, not by automatically replacing the DEF/AdBlue temperature sensor.
The most useful first steps are to check the live reductant temperature reading, retrieve freeze-frame and related fault codes, inspect the connector and wiring, and then test the reference voltage, sensor ground, signal circuit, and sensor resistance according to the manufacturer's specifications.
If the sensor is integrated into the DEF/AdBlue tank module, the complete module should only be replaced after testing confirms that the integrated sensor or internal circuitry has failed.
Ignoring P2042 can eventually result in DEF/AdBlue warnings, incorrect reductant heating or dosing, reduced SCR efficiency, increased NOx emissions, additional aftertreatment faults, reduced engine power, and operating restrictions.
The correct repair is to identify the actual source of the circuit malfunction, repair the wiring or connector or replace the confirmed defective sensor/module, perform any required calibration or initialization, clear the code, and verify that the reductant temperature signal operates normally.
