P2042
Reductant Temperature Sensor A Circuit
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.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
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)
Common causes
Ordered by how often each turns out to be the cause.
- Sensor element failedCommon
- Harness faultPossible
Parts involved
- Reductant Temperature Sensor
- Reductant Tank
P2042 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.