Faulty reductant temperature sensor; Damaged or corroded wiring or connector at the sensor; Open or short circuit in the sensor signal or ground circuit; Failed DEF heater causing incorrect temperature readings; Contaminated or degraded DEF fluid affecting sensor performance; Powertrain control module software issue or internal fault

DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P2042

P2042Reductant Temperature Sensor Circuit

Q: What does code P2042 mean?

Reductant Temperature Sensor Circuit. P2042 indicates that the powertrain control module has detected an abnormal signal from the reductant temperature sensor, which monitors the temperature of the diesel exhaust fluid (DEF) used in selective catalytic reduction systems. The computer compares the sensor's voltage or resistance against expected values and flags the code when the reading falls outside the valid range or behaves erratically.

P2042 indicates that the powertrain control module has detected an abnormal signal from the reductant temperature sensor, which monitors the temperature of the diesel exhaust fluid (DEF) used in selective catalytic reduction systems. The computer compares the sensor's voltage or resistance against expected values and flags the code when the reading falls outside the valid range or behaves erratically.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering and auxiliary emission controls

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
Reduced engine power or limp mode in some vehicles
A message on the driver information display warning about the DEF system
Increased DEF consumption or failure to inject DEF
Rough running or poor fuel economy in severe cases
Possible no-start condition if the vehicle is programmed to prevent operation with a faulty DEF system

Common causes, most likely first

Faulty reductant temperature sensor
Damaged or corroded wiring or connector at the sensor
Open or short circuit in the sensor signal or ground circuit
Failed DEF heater causing incorrect temperature readings
Contaminated or degraded DEF fluid affecting sensor performance
Powertrain control module software issue or internal fault

How to diagnose and fix it

Use a scan tool to retrieve the freeze-frame data and verify the sensor reading against ambient temperature after a cold soak
Visually inspect the sensor, connector, and wiring for damage, corrosion, or loose pins
Measure the sensor resistance with a multimeter and compare to the manufacturer's specification
Check the sensor signal voltage at the PCM connector to rule out wiring faults
Test the DEF heater circuit if the sensor is integrated with the heater
Replace the reductant temperature sensor if it fails electrical testing
Clear the code and perform a road test to confirm the repair

What the code does not always mean

A P2042 can be set by a weak battery or charging system issue that causes low voltage to the sensor, so always check battery health first.
The reductant temperature sensor is often part of the DEF tank heater assembly, and replacing just the sensor may not be possible on some designs.
Contaminated DEF fluid from using the wrong type or diluting with water can cause sensor readings to drift without a hard failure.
In cold climates, the DEF heater is critical, and a failed heater can cause the sensor to read incorrectly and set this code.
Some vehicles require a relearn or adaptation reset after sensor replacement, otherwise the code may return even with a good part.

Can I keep driving with P2042?

You can usually continue driving with P2042, but avoid long trips or heavy loads until diagnosed, as the vehicle may enter limp mode or refuse to restart if the DEF system fault escalates. Do not ignore the warning light, because a failed sensor can lead to improper DEF dosing and eventual aftertreatment damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.