P0548 Exhaust Gas Temperature (EGT) Sensor Circuit Low (Bank 2, Sensor 1)

The P0548 trouble code indicates that the engine control module (ECM/PCM) has detected an exhaust gas temperature (EGT) sensor signal that is lower than the expected electrical range on Bank 2, Sensor 1.

The important point is that “Circuit Low” refers primarily to the electrical signal, not automatically to exhaust gas temperature being too low.

Depending on the vehicle, the EGT sensor may be used to monitor exhaust temperature for protection of the turbocharger, catalytic converter, diesel particulate filter (DPF), exhaust gas recirculation system, or other emissions components.

P0548 therefore deserves careful diagnosis because an incorrect EGT reading can cause the ECU to change fueling, regeneration strategy, boost control, or other protective functions.

What Is an EGT Sensor?

An Exhaust Gas Temperature (EGT) sensor measures the temperature of exhaust gases at a specific location in the exhaust system.

The sensor is generally installed in a position where exhaust temperature is particularly important.

Depending on the engine, EGT sensors can be located:

• Before a turbocharger

• After a turbocharger

• Before a catalytic converter

• After a catalytic converter

• Before a DPF

• After a DPF

• Near an exhaust manifold

• In other sections of the exhaust system

The exact purpose of Bank 2, Sensor 1 depends on the engine design.

On a V-type engine, Bank 2 generally refers to the cylinder bank opposite Bank 1. On some engines, however, the manufacturer-specific sensor numbering and exhaust layout must be checked before assuming the physical location.

What Does “Sensor 1” Mean?

“Sensor 1” generally identifies the first EGT sensor in the relevant bank or exhaust monitoring sequence.

It does not necessarily mean the first temperature sensor physically visible when looking at the engine.

This is important because modern exhaust systems can contain several temperature sensors.

A vehicle may have multiple EGT sensors monitoring different locations, such as:

• Turbocharger inlet

• Turbocharger outlet

• Catalyst inlet

• Catalyst outlet

• DPF inlet

• DPF outlet

The wiring diagram and manufacturer's component-location information should be used to identify the actual Bank 2 Sensor 1.

What Does “Circuit Low” Mean?

P0548 means the ECU is detecting an electrical signal below the expected range from the Bank 2 Sensor 1 EGT circuit.

This can happen for several reasons.

For example:

• Signal wire shorted to ground

• Sensor internally shorted

• Damaged wiring

• Connector problem

• Incorrect sensor resistance

• Reference-voltage problem

• Sensor ground problem, depending on design

• Harness contamination

• Control-module fault

A low electrical signal does not automatically prove that the exhaust itself is cold.

This distinction is particularly important when diagnosing EGT sensors.

How EGT Sensors Work

Many EGT sensors used in automotive applications are thermistors, meaning their electrical resistance changes as temperature changes.

The ECU supplies an electrical circuit and interprets the resulting signal to calculate exhaust temperature.

However, the exact sensor technology varies.

Some systems use:

• NTC thermistors

• PTC-type sensors

• Thermocouples

• Dedicated temperature-sensing circuits

Because of these differences, it is not correct to apply one universal resistance or voltage value to every EGT sensor.

The manufacturer's specifications should always be used.

Why Does the ECU Care About Exhaust Temperature?

Exhaust temperature is important because excessive heat can damage expensive components.

Depending on the vehicle, EGT information can be used to protect:

• Turbochargers

• Catalytic converters

• DPFs

• Exhaust valves

• Exhaust manifolds

• Emissions-control components

On diesel engines, EGT information can also be important during DPF regeneration.

During regeneration, exhaust temperature must reach a sufficiently high level to help burn accumulated soot.

If the ECU cannot trust the EGT reading, it may modify or stop certain strategies to protect the exhaust system.

Symptoms of P0548

The symptoms depend on the vehicle and the way EGT information is used.

Possible symptoms include:

• Check Engine Light

• Reduced engine power

• Limp mode

• Poor acceleration

• Increased fuel consumption

• DPF regeneration problems

• Regeneration being disabled

• Turbocharger control changes

• Increased exhaust emissions

• Rough or altered engine operation

• Longer regeneration cycles

• Other exhaust-temperature-related fault codes

Some vehicles may show almost no noticeable drivability symptoms.

The ECU may simply substitute a default temperature value and continue operating with limited functionality.

Common Causes of P0548

1. Failed EGT sensor

The sensor may internally fail and produce an abnormally low signal.

A sensor that has been exposed to extremely high exhaust temperatures for a long period can eventually deteriorate.

2. Signal wire shorted to ground

This is an important possibility with a low-input code.

If the signal circuit is unintentionally connected to ground, the ECU may interpret the resulting low voltage as an abnormally low EGT signal.

3. Damaged wiring

EGT sensor wiring operates in one of the hottest areas of the vehicle.

The harness can suffer from:

• Heat damage

• Brittle insulation

• Melting

• Chafing

• Vibration

• Mechanical contact with exhaust components

A wiring problem can therefore be more likely than on sensors located in cooler areas of the engine bay.

4. Damaged connector

The connector can be affected by:

• Heat

• Corrosion

• Moisture

• Poor terminal tension

• Contamination

• Broken locking tabs

A connector that has been repeatedly exposed to high temperatures can lose its mechanical and electrical integrity.

5. Reference or sensor circuit problem

Depending on the sensor architecture, a problem with the reference or sensing circuit can cause the ECU to interpret the EGT signal incorrectly.

6. Exhaust-system work

Recent repairs can be relevant.

For example, if the exhaust, turbocharger, DPF, catalytic converter, or sensor has recently been removed, the EGT harness may have been:

• Stretched

• Pinched

• Incorrectly routed

• Connected incorrectly

• Damaged during installation

The timing of the code can provide a useful clue.

7. Incorrect sensor

An EGT sensor that looks physically identical may still have different electrical characteristics.

Installing the wrong sensor can produce implausible temperature readings and trigger circuit-related faults.

8. ECM/PCM problem

A control-module fault is possible, but it should be considered only after the sensor and its complete circuit have been tested.

How to Diagnose P0548

The first step should be to scan the vehicle for all stored and pending codes.

Do not diagnose P0548 in isolation.

Look for related codes involving:

• Other EGT sensors

• DPF

• Turbocharger

• Catalyst

• Fuel system

• Exhaust aftertreatment

• Reference voltage

• Sensor circuits

If several sensors suddenly show abnormal readings, a common electrical problem may exist.

Check the Live EGT Data

With a scan tool capable of displaying EGT data, compare the Bank 2 Sensor 1 reading with:

• Engine coolant temperature

• Other EGT sensors

• Intake temperature

• Operating conditions

When the engine is cold and has been sitting long enough, the EGT reading should generally be reasonably consistent with the surrounding temperatures.

If the scan tool shows an obviously implausible value while the engine is cold, that provides a strong clue.

The most useful observation is often what happens as the engine warms and load changes.

Check the Electrical Signal

The next step is to identify the exact sensor circuit.

Do not assume every EGT sensor uses the same voltage or resistance characteristics.

Depending on the design, the technician may need to test:

• Signal voltage

• Sensor resistance

• Reference circuit

• Ground circuit

• Continuity

• Short-to-ground condition

• Short-to-voltage condition

The manufacturer's wiring diagram and specifications are essential.

Test the Sensor Resistance When Applicable

If the vehicle uses a resistive EGT sensor and the manufacturer specifies a resistance test, measure the sensor at a known temperature.

Then compare the measured resistance with the manufacturer's temperature/resistance specification.

A sensor that has a resistance far outside specification can be identified without immediately replacing other components.

However, resistance testing should only be used when appropriate for that particular sensor design.

A thermocouple-based EGT system, for example, should not be diagnosed using the same procedure as a thermistor.

Check for a Short to Ground

Because P0548 is a low-input code, a short to ground is an important diagnostic possibility.

The technician should identify the signal circuit and test it according to the manufacturer's procedure.

If the signal circuit is unintentionally grounded, the ECU may interpret the condition as an extremely low temperature or invalid EGT signal.

This can produce P0548 even when the sensor itself is perfectly healthy.

Inspect the Harness Near the Exhaust

This is one of the most important physical inspections.

Follow the wiring from the EGT sensor toward the main harness.

Look for:

• Melted insulation

• Hardened insulation

• Burn marks

• Chafing

• Broken wires

• Incorrect routing

• Wiring touching the exhaust

• Loose retaining clips

EGT sensor wiring is particularly vulnerable because the exhaust system can reach extremely high temperatures.

A wire can appear intact while the conductor inside has been damaged by repeated heat cycles.

Compare the EGT Sensors

If the vehicle has multiple EGT sensors, comparing their live readings can be extremely useful.

For example, if the engine is fully warmed and several sensors show plausible temperature changes while Bank 2 Sensor 1 remains fixed at an unrealistic value, the fault becomes much easier to isolate.

However, the sensors are positioned at different points in the exhaust, so their temperatures should not necessarily be identical.

The comparison should be based on whether their behavior is plausible for their respective locations.

Can P0548 Mean the Exhaust Temperature Is Actually Too Low?

It can, but the code itself does not prove that.

A genuinely low exhaust temperature could result from an engine or operating condition, but “Circuit Low” directs the initial investigation toward the electrical signal.

The technician should first establish whether the ECU is receiving an electrically valid signal.

If the electrical signal is correct, then actual exhaust temperature and engine operating conditions can be investigated.

Can P0548 Cause DPF Problems?

On diesel vehicles equipped with a DPF, it potentially can.

The ECU needs reliable temperature information to control and protect the aftertreatment system.

If the Bank 2 Sensor 1 EGT signal is considered invalid, the ECU may alter regeneration strategy or use a fallback strategy.

Depending on the vehicle, this can contribute to:

• Failed regeneration

• Incomplete regeneration

• Increased soot accumulation

• DPF warning messages

• Reduced engine performance

However, P0548 does not automatically mean that the DPF itself is defective.

Can P0548 Cause Limp Mode?

Yes, on some vehicles.

If the ECU cannot reliably determine exhaust temperature, it may limit engine operation to protect the turbocharger, catalyst, DPF, or other components.

The severity of the response depends on the manufacturer's strategy.

Some vehicles will continue operating almost normally, while others may substantially reduce available power.

Should the EGT Sensor Be Replaced Immediately?

No.

A low-input code can be caused by the wiring or electrical circuit just as easily as by the sensor itself.

A sensible diagnostic sequence is:

Scan all codes → check live EGT data → inspect the sensor and wiring → identify the circuit type → test signal/reference/ground as applicable → test the sensor → check for shorts and opens → verify actual exhaust behavior if necessary.

Only then should the failed component be replaced.

Important Diagnostic Detail: Cold-Start Testing

A cold engine provides an excellent opportunity to identify an implausible EGT reading.

After the vehicle has been parked long enough for the exhaust system to cool, compare the Bank 2 Sensor 1 reading with the other relevant temperature sensors.

If the engine is completely cold but the scan tool reports an extreme or physically implausible EGT value, the sensor circuit deserves immediate attention.

This is often more informative than checking the sensor after the exhaust has become extremely hot.

Final Diagnostic Summary

P0548 means the ECM/PCM has detected a low electrical input from the Exhaust Gas Temperature (EGT) Sensor Bank 2, Sensor 1 circuit.

The code does not automatically mean that the exhaust gas temperature itself is too low.

The most likely areas to investigate are the EGT sensor, signal wiring, connector, reference/sensing circuit, and possible short to ground. Because the sensor operates in an extremely hot environment, heat-damaged wiring and connectors deserve particular attention.

Start with all stored and pending codes and the live EGT data. Identify the exact Bank 2 Sensor 1 location and sensor technology, then test the circuit according to the manufacturer's specifications.

If the electrical signal is correct, only then should the actual exhaust temperature and related components such as the turbocharger, catalyst, or DPF be investigated.

The key diagnostic question is:

Is the EGT sensor actually reporting a low temperature, or is an electrical fault forcing the ECU to see a low signal?

That distinction is critical when diagnosing P0548 and can prevent an unnecessary EGT sensor, DPF, turbocharger, or exhaust-system replacement.