What it means
A general electrical fault is reported on reductant (DEF) temperature sensor A — used to compensate dosing for fluid viscosity and to enable tank heating in winter.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant temperature sensor A reports the temperature of DEF in the after-treatment system. The control module uses that reading when managing dosing and tank heating; P2042 sets when the sensor circuit has an electrical fault, without specifying whether the signal is too low or too high.
Common causes
Most likely first.
- Sensor element failed
- Harness fault
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
How to diagnose it
- Read freeze-frame data and live reductant temperature to see whether sensor A reports a plausible value or an electrical fault while the code is present.
- Inspect sensor A's connector and harness for loose terminals, corrosion, moisture, damaged insulation, or a disconnected plug.
- Check the sensor circuit wiring and connector terminals for opens, shorts, or poor contact using the manufacturer's wiring information.
- Test the temperature sensor A and its circuit against the manufacturer's specification to distinguish a failed sensor from a harness fault.
- After repairing the fault, clear the code and verify that the temperature reading responds plausibly during operation.
Common fixes
- Reconnect or repair a loose, corroded, or damaged sensor A connector.
- Repair an open or short in the reductant temperature sensor circuit.
- Replace reductant temperature sensor A if it fails the manufacturer's electrical test.
Can I keep driving?
If the vehicle runs normally, short trips may be possible, but address the DEF or NOx warning promptly. Incorrect temperature information can interfere with dosing or cold-weather heating, and the fault may eventually trigger a no-start countdown or cause an emissions test failure.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.