P21C8Reductant Control Module Power Relay/Relays Control Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P21C8 means the control line of the DEF module's power relay, or relays, sits too close to ground. No manufacturer documents P21C8 publicly. Ford's 2023 diesel summary only shows the open-circuit sibling P21C7, judged by comparing the commanded relay state with the voltage on the switched side; a low-side code follows the same idea with a pin voltage below the expected window. What a short to ground does depends on the driver: if the module switches the coil on its ground side, the relay may stay energised and drain the battery after switch-off; if it switches the supply side, the relay never pulls in and the SCR system loses power. The engine module pair P0685/P0686 is the closest analogy. P21C9 is the high-side fault.
- What you may notice
- Check engine light on
- DEF system without power or permanently powered
- Flat battery after the vehicle has been parked
- Likely causes
- LikelyControl wire chafed and shorted to ground
- PossibleShorted relay coil
- PossibleMoisture in relay socket or module connector
- RareOutput stage in the controlling module faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €30 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.