P21C2 Reductant Heater Relay Control Circuit/Open
What does P21C2 mean?
P21C2 — Reductant Heater Relay Control Circuit/Open. This is an OBD-II diagnostic trouble code a scanner retrieves when your vehicle's computer detects reductant heater relay control circuit/open. The level of risk depends on how the vehicle is currently running and how long you've been driving with the light on.
Common causes of P21C2
- Faulty or contaminated sensor in the fuel/air metering circuit
- Damaged or shorted wiring or connector for the affected circuit
- Vacuum leak or intake system restriction
- Failing fuel pump, injector, or fuel pressure regulator
How to fix P21C2
These are general steps for this system. The exact cause depends on your vehicle — confirm with live scan data before replacing parts.
- Read freeze frame data and related codes with an OBD-II scanner
- Inspect the affected sensor wiring and connector for damage
- Compare live sensor readings (fuel trim, airflow, temperatures) to manufacturer specs
- Smoke test the intake system if lean or vacuum conditions are suspected
- Replace the failed component after confirming it with live data