P21C2Reductant Heater Relay Control Circuit/Open
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P21C2 concerns the control line of the relay that supplies current to the DEF heaters. The control module drives the relay coil through an output stage and checks whether the circuit reacts; an open circuit means the coil, its wiring or a connector has lost continuity, so the heaters cannot be energised. Ford documents the code in its 2023 diesel summary: the check runs continuously while heating is requested, and with the relay stage on and every heater switched off, a heater current below 0.2 A sets P21C2 within at most 5 s. P21C3 and P21C4 are the low and high faults of the same line, P21C7 covers the power relay of the module itself. In frost the fluid may stay frozen, so dosing starts late and DEF warnings can follow.
- What you may notice
- Check engine light on
- DEF system warning, especially at low ambient temperature
- Frozen DEF thaws slowly, dosing begins late after a cold start
- Likely causes
- LikelyRelay coil burnt open or relay not seated in its socket
- PossibleBroken control wire between module and relay
- PossibleCorroded or loose terminals at relay box or module connector
- RareFaulty output stage in the controlling module
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €20 – €400
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.