P2042
Reductant Temperature Sensor A Circuit
Auxiliary emissions control
Generic OBD-II
Medium
What it means
P2042 means the engine computer sees a circuit fault at reductant temperature sensor A, part of the diesel SCR/DEF (exhaust-fluid) emissions system. This sensor monitors DEF temperature so the system can dose correctly and avoid freezing. The engine runs normally at first, but unresolved emissions faults can trigger a power derate, so it's worth fixing promptly.
Medium
Can I keep driving? Yes for now — but this is part of the diesel emissions (SCR/DEF) system, and an unfixed fault can lead to a power derate; repair soon.
Common symptoms
What you'll notice
- Check-engine light on
- SCR/emissions warning message and possible countdown to reduced power
- Usually no change in how the engine runs at first
Common causes
Most likely first
- Faulty reductant (DEF) temperature sensor ALikely
- Open or short in the reductant temperature sensor wiringCommon
- Corroded or loose connector at the sensorCommon
- Failed powertrain control module (PCM)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and inspect the reductant temperature sensor connector and wiring for damage or corrosion.
- Check the sensor circuit for opens or shorts against the wiring diagram; repair as needed.
- Compare the sensor's resistance/reading to spec; replace the reductant temperature sensor if out of range.
- Clear the code and confirm the reading is plausible after a drive cycle.
FAQ
Quick answers
Is it safe to drive with P2042?+
Short-term yes, but the DEF/SCR system can force reduced power if emissions faults go unresolved, so don't put the repair off.
Can I just clear P2042?+
It returns until the sensor or wiring is fixed, and clearing it won't stop an emissions derate countdown.
Related codes
Often seen with P2042
Read & clear P2042 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX