P20C9Reductant Control Module Requested MIL Illumination
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P20C9 is a forwarding code. Some diesel trucks use a separate DEF or SCR control module with its own fault memory; when that module detects an emission-relevant fault it asks the engine module to switch on the check engine lamp, and the engine module records P20C9. In GM bulletin 17-NA-366 for the 2018 Chevrolet Low Cab Forward 6500XD, every SCR code stays in the DEF module except this one, which the ECM stores and which turns the MIL on. Isuzu's 2011 N-Series with the 5.2 L diesel sets it automatically as soon as the DEF module logs a NOx sensor code such as P2201, P2206 or P2207 (special policy CB14-L-001). A generic reader therefore often shows only P20C9; the actual fault has to be read from the DEF module itself. Comparable request codes exist for other modules, for example P0700 for the transmission.
- What you may notice
- Check engine light with only P20C9 visible on a generic scan tool
- DEF or SCR warning lamp, depending on the underlying fault
- Possible power derate if the underlying SCR fault triggers inducement
- Likely causes
- LikelyEmission-relevant SCR fault stored in the DEF control module (heater, pump, pressure, sensor)
- PossibleFailed upstream NOx sensor reported by the DEF module
- RareOutdated or mismatched software between engine and DEF control module
- 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: €50 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.