What Is a Reductant Temperature Sensor? Symptoms, Causes, Diagnosis and Solutions

The Reductant Temperature Sensor is an important component of modern diesel emission control systems equipped with Selective Catalytic Reduction (SCR) technology.

It measures the temperature of the reductant fluid, commonly known as AdBlue or Diesel Exhaust Fluid (DEF), and sends this information to the vehicle's engine control module or SCR control module.

The temperature of AdBlue is important because the fluid behaves differently depending on environmental and operating conditions. It can freeze at low temperatures, while excessive temperatures can affect the condition of the fluid and the operation of the SCR system.

The control module uses the temperature information to manage the AdBlue system correctly, including heating, dosing and diagnostic functions.

A faulty reductant temperature sensor can cause AdBlue or SCR warning messages, diagnostic trouble codes, incorrect system operation and, in some vehicles, reduced engine performance or a restart restriction.

What Is a Reductant Temperature Sensor?

The Reductant Temperature Sensor is a temperature-sensing component used in the AdBlue/DEF system.

It measures the temperature of the reductant fluid inside the tank or, depending on the system design, at another location within the reductant supply system.

The sensor sends an electrical signal to the control module.

The control module uses this information to determine whether the AdBlue temperature is within the expected operating range.

The sensor may be integrated into:

• The AdBlue/DEF tank assembly

• The reductant delivery module

• The dosing module

• A reductant level and temperature sensor assembly

The exact design depends on the vehicle manufacturer and SCR system.

What Is Reductant?

In an SCR-equipped diesel vehicle, reductant refers to the fluid used to reduce nitrogen oxide emissions.

In many European vehicles, this fluid is known as AdBlue.

In other markets, particularly North America, it is commonly called Diesel Exhaust Fluid (DEF).

The fluid is injected into the exhaust system and participates in the SCR chemical process.

The reductant temperature sensor does not measure exhaust temperature. It measures the temperature of the reductant fluid.

Why Is Reductant Temperature Important?

AdBlue is a water-based urea solution and its physical properties change with temperature.

One particularly important characteristic is that AdBlue can freeze at approximately -11°C (12°F).

Modern SCR systems are therefore designed with strategies to deal with freezing conditions.

The control module needs accurate temperature information to determine the condition of the fluid and manage the system appropriately.

Temperature information can influence:

• AdBlue heating

• Pump operation

• Dosing strategy

• System diagnostics

• Freeze protection

• Fault monitoring

An incorrect temperature signal can therefore affect the operation of the entire SCR system.

How Does a Reductant Temperature Sensor Work?

The sensor is commonly based on a thermistor.

A thermistor changes its electrical resistance according to temperature.

The control module supplies a reference voltage to the sensor circuit and monitors the resulting signal.

As the temperature changes, the electrical signal changes accordingly.

The ECM or SCR control module converts this signal into a temperature value.

The exact electrical characteristics vary between sensor designs.

For this reason, resistance or voltage should always be compared with the manufacturer's specifications.

Where Is the Reductant Temperature Sensor Located?

The sensor location depends on the vehicle.

In many systems, the temperature sensor is installed inside the AdBlue tank assembly.

It may be integrated with other components such as:

• Reductant level sensor

• Reductant heater

• Pump

• Delivery module

Some systems use a combined level-temperature sensor assembly.

Because the sensor can be integrated into the tank or delivery module, replacing it may sometimes require servicing or replacing a larger assembly.

What Does the Reductant Temperature Sensor Do?

The sensor provides the control module with real-time information about the AdBlue temperature.

This information allows the control system to determine whether:

• The reductant is frozen or near freezing conditions

• The heating system needs to operate

• The fluid is within its expected temperature range

• The dosing system can operate normally

• The temperature signal is plausible

• There is a potential sensor or wiring fault

Without reliable temperature information, the SCR system may not be able to manage the reductant system correctly.

Symptoms of a Faulty Reductant Temperature Sensor

A defective reductant temperature sensor can cause several symptoms.

Common signs include:

• Check Engine Light

• AdBlue warning light

• SCR system warning

• Reductant system fault message

• Incorrect AdBlue temperature reading

• Diagnostic trouble codes

• Problems with AdBlue heating

• Incorrect dosing strategy

• Reduced engine power in some vehicles

• Emissions-related warnings

• Restart countdown or restriction in some vehicles

The vehicle may continue to drive normally at first.

This is because the sensor is part of the emissions-control system rather than the engine's primary fuel injection system.

Check Engine Light

A temperature sensor fault can trigger the Check Engine Light when the control module detects an electrical or plausibility problem.

For example, the module may detect:

• Temperature signal too high

• Temperature signal too low

• Open circuit

• Short circuit

• Implausible temperature

• Signal that does not change as expected

The exact diagnostic trouble code depends on the vehicle and SCR system.

AdBlue Warning Message

A defective temperature sensor may cause the vehicle to display an AdBlue or SCR warning.

Depending on the manufacturer, the dashboard may display messages such as:

• AdBlue system fault

• SCR system malfunction

• Reductant temperature sensor fault

• DEF system fault

• Emissions system fault

• AdBlue quality or system warning

These messages do not automatically mean that the sensor itself has failed.

The wiring, connector, heating system, control module or reductant delivery module may also be responsible.

Incorrect Reductant Temperature Reading

One of the clearest indications of a sensor problem is an unrealistic temperature reading.

For example, if the vehicle has been parked overnight in moderate conditions but the scan tool reports an extremely low or extremely high AdBlue temperature, the sensor signal should be investigated.

Similarly, if the temperature remains fixed at one value despite significant changes in the surrounding conditions, there may be a sensor or circuit problem.

Problems With AdBlue Heating

AdBlue systems often include heating elements to prevent the fluid from remaining frozen in cold conditions.

The control module uses temperature information to determine when heating is required.

If the sensor reports an incorrect temperature, the control system may:

• Activate the heater unnecessarily

• Fail to activate the heater when required

• Delay heating

• Record a heating-system fault

However, a heating problem does not automatically mean that the temperature sensor is defective.

The heater, wiring, relay, fuse and control module should also be checked.

Reductant Temperature Sensor Circuit Problems

Electrical faults are among the most common reasons for temperature sensor-related diagnostic codes.

Possible problems include:

• Open circuit

• Short circuit to ground

• Short circuit to voltage

• Damaged wiring

• Corroded connector

• Loose terminal

• Water intrusion

• Internal sensor failure

Because the sensor is often located near the AdBlue tank, its wiring and connectors can be exposed to moisture, road salt, dirt and temperature changes.

Common Causes of Reductant Temperature Sensor Failure

Several factors can cause the sensor to malfunction.

Internal sensor failure

The thermistor or electronic components inside the sensor may fail.

Damaged wiring

The harness can become damaged through vibration, heat or physical contact.

Connector corrosion

Moisture and road contamination can corrode electrical terminals.

Water intrusion

A damaged connector seal can allow moisture to enter the electrical connection.

Extreme temperatures

Repeated exposure to freezing and high temperatures can contribute to component deterioration.

AdBlue system contamination

Incorrect or contaminated reductant can damage components within the system.

Tank assembly failure

If the sensor is integrated into the reductant tank module, a fault in the assembly can affect temperature measurement.

Can Frozen AdBlue Damage the Temperature Sensor?

Freezing does not necessarily mean that the temperature sensor itself has failed.

AdBlue is designed to freeze at low temperatures and the vehicle's SCR system is designed to manage this condition.

The system normally uses heating strategies to return the fluid to a usable state.

However, repeated freezing, incorrect fluid, physical damage or problems within the tank assembly can contribute to component failures.

A frozen AdBlue tank should therefore not be treated as proof of a defective temperature sensor.

Can Low AdBlue Level Cause a Temperature Sensor Fault?

Normally, a low fluid level does not directly mean that the temperature sensor has failed.

However, the sensor may be integrated into a larger level and temperature sensing assembly.

In such systems, a problem with the assembly can affect multiple measurements.

A diagnostic scan should therefore be used to determine which parameter is actually outside the expected range.

Can Poor-Quality AdBlue Cause Sensor Problems?

Yes.

The SCR system is designed for reductant fluid that meets the appropriate specification.

Contaminated or incorrect fluid can cause problems with:

• Temperature sensors

• Level sensors

• Pumps

• Injectors

• Supply lines

• SCR components

Diesel fuel, water, coolant or other substances should never be added to the AdBlue tank.

Reductant Temperature Sensor vs Exhaust Temperature Sensor

These two sensors are easy to confuse because both are related to temperature.

The reductant temperature sensor measures the temperature of AdBlue/DEF.

The exhaust gas temperature sensor measures the temperature of the exhaust gases.

They have completely different functions and are normally located in different parts of the vehicle.

The exhaust temperature sensor is generally installed in the exhaust system.

The reductant temperature sensor is normally installed in the AdBlue tank or reductant delivery system.

Reductant Temperature Sensor vs NOx Sensor

The NOx sensor measures nitrogen oxide concentration in the exhaust.

The reductant temperature sensor measures AdBlue/DEF temperature.

The SCR control system may use information from both sensors, but they perform completely different functions.

A NOx sensor fault should not be confused with a reductant temperature sensor fault.

How Is a Reductant Temperature Sensor Diagnosed?

Diagnosis should begin with a diagnostic scan.

The technician should read the stored fault codes and examine the live data.

Important checks may include:

• Reductant temperature reading

• Ambient temperature

• Reductant level

• Heater operation

• Sensor voltage

• Reference voltage

• Ground circuit

• Wiring continuity

• Connector condition

The reported temperature should be compared with the actual conditions.

Checking Live Temperature Data

Live data can provide a quick indication of whether the sensor is behaving logically.

For example, when the vehicle has been sitting for a long period, the reductant temperature should generally be reasonably consistent with the surrounding conditions.

As the vehicle operates and the AdBlue system warms up, the temperature reading should respond appropriately.

A reading that remains fixed, jumps unexpectedly or is physically implausible may indicate a sensor or circuit problem.

Testing the Sensor Electrically

Depending on the sensor design, a technician may test:

• Reference voltage

• Ground

• Signal voltage

• Resistance

• Wiring continuity

• Short circuits

• Open circuits

If the sensor uses a thermistor, its resistance should change with temperature.

The exact resistance-temperature relationship must be obtained from the manufacturer's technical information.

Using a generic resistance value can lead to an incorrect diagnosis.

Testing the Reductant Heater

If the temperature sensor indicates a problem in freezing conditions, the heater should also be checked.

The technician may inspect:

• Heater power supply

• Ground

• Fuse

• Relay

• Wiring

• Heater resistance

• Control signal

• Actual temperature increase

A sensor fault and a heater fault can produce related symptoms, so both systems may need to be tested.

Can a Reductant Temperature Sensor Be Repaired?

In most cases, the sensor itself is not repaired internally.

If the sensing element has failed, replacement is generally the appropriate solution.

However, if the problem is caused by:

• Damaged wiring

• Corroded connector

• Loose terminal

• Poor electrical connection

the wiring or connector may be repairable without replacing the sensor.

This is why electrical testing should be performed before installing a new component.

Does the Sensor Need to Be Replaced With the Whole AdBlue Tank?

Not necessarily.

The answer depends on the vehicle's design.

On some vehicles, the reductant temperature sensor is a separate serviceable component.

On others, it may be integrated into the tank, pump or delivery module.

If the manufacturer does not provide the sensor as a separate replacement component, the complete module may need to be serviced.

The correct parts configuration should therefore be verified using the vehicle's VIN and manufacturer parts information.

Can You Drive With a Faulty Reductant Temperature Sensor?

The vehicle may continue to operate with a faulty sensor.

However, continued driving can result in persistent SCR warnings and eventually additional restrictions depending on the vehicle's emission-control strategy.

The vehicle may also be unable to correctly manage reductant heating and dosing under certain conditions.

For this reason, the fault should be diagnosed rather than ignored.

Can a Faulty Temperature Sensor Prevent Engine Starting?

The temperature sensor itself does not normally control the diesel fuel injection system.

However, a persistent SCR system fault can eventually contribute to a restart restriction on vehicles that use such a strategy.

This means the sensor may indirectly be associated with a no-restart condition after the system has reached the final stage of an emissions-related warning.

The exact strategy depends on the manufacturer and vehicle.

How to Prevent Reductant Temperature Sensor Problems

Good maintenance practices can help reduce SCR system problems.

• Use only the correct specification of AdBlue/DEF.

• Do not mix other fluids with AdBlue.

• Keep the filler area clean.

• Protect electrical connectors from contamination.

• Repair damaged wiring promptly.

• Do not ignore SCR warning messages.

• Address AdBlue leaks quickly.

• Follow the manufacturer's maintenance procedures.

• Use appropriate replacement components.

The entire reductant system should be maintained rather than focusing only on the temperature sensor.

Frequently Asked Questions

What does a reductant temperature sensor measure?

It measures the temperature of the AdBlue/DEF fluid in the vehicle's SCR system.

Is the reductant temperature sensor the same as the exhaust temperature sensor?

No. The reductant temperature sensor measures AdBlue temperature, while the exhaust temperature sensor measures exhaust gas temperature.

Can a faulty reductant temperature sensor cause an AdBlue warning?

Yes. An incorrect temperature signal can cause the SCR control system to detect a fault and display an AdBlue or emissions warning.

Can a bad temperature sensor cause reduced engine power?

It can indirectly contribute to reduced power if the vehicle's SCR control strategy activates a reduced-performance mode because of a persistent emissions fault.

Can a faulty sensor cause an AdBlue heater problem?

Yes. The control module relies on temperature information to determine how the heating system should operate. However, the heater itself and its electrical circuit must also be tested.

Can contaminated AdBlue damage the temperature sensor?

Contaminated or incorrect reductant can damage or interfere with SCR system components, including sensors and dosing components.

Does a temperature sensor fault always mean the sensor must be replaced?

No. Wiring, connectors, power supply, ground, the reductant module and other components can cause similar fault codes.

Final Thoughts

The Reductant Temperature Sensor is a small but important component of the modern AdBlue/DEF and SCR system.

Its job is to measure the temperature of the reductant fluid and provide this information to the vehicle's control module.

This information is used to manage AdBlue heating, dosing, freeze protection and system diagnostics.

A faulty sensor can cause AdBlue warnings, SCR system faults, Check Engine Light illumination, incorrect temperature readings, heating problems and, in some vehicles, reduced engine power or a restart restriction.

However, replacing the sensor based solely on a diagnostic trouble code is not always the correct solution.

The sensor circuit, wiring, connectors, reductant heater, tank module, AdBlue quality and control system should also be considered.

The most reliable diagnosis combines fault-code analysis, live-data evaluation and electrical testing.

Correct diagnosis can prevent unnecessary replacement of expensive AdBlue tank or dosing-system components and help ensure that the SCR system continues to operate correctly.