Powertrain
generic / SAE
P21C4
Reductant Heater Relay Control Circuit High
DEF heater relay control line stuck high — short to B+. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
SystemPowertrain
Typegeneric / SAE
GroupP2
Range / number1 / C4
Possible causes
- Short to B+ high
- Connector pins corroded - high contact resistance low
- Supply wire pinched against B+ source low
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)
Inspection / repair
- Difficulty: medium
- DIY: possible
- Estimated cost: 50–400 €
- Estimated time: 0.5–2 h
- Component: reductant heater relay
- MIL / check engine light
- emissions-relevant
Sources
References
sae:J2012