P21C9Reductant Control Module Power Relay/Relays Control Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P21C9 is stored when the control line of the DEF module's power relay, or relays, carries more voltage than the driver state allows, typically because the wire touches a battery feed or the driver can no longer pull it down. No manufacturer documents P21C9 publicly. The closest published logic is Ford's 2023 diesel summary for P21C7, which flags voltage on the switched side while the output stage is off; a high-side code adds the specific diagnosis of a short to supply on the control pin. Depending on the driver layout the relay then either never releases, keeping pump and heaters live with the engine off, or never closes. P0687 is the equivalent code for the engine module relay, and a late drop-out after key-off is reported as P20EB.
- What you may notice
- Check engine light on
- DEF pump or module audible or active with the engine off
- Battery drained overnight
- Likely causes
- LikelyControl wire shorted to a battery-voltage circuit
- PossibleRelay contacts welded, switched side stays live
- RareDriver in the controlling module shorted
- 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.