P20C8Reductant Heater D 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
- Diesel exhaust fluid freezes at about -11 °C, so SCR systems carry several electric heating elements in the tank, pump and lines; which one a maker calls heater D varies. P20C8 reports that the control circuit of heater D reads high, meaning the output or its wiring has contact with battery voltage and the element can no longer be switched as commanded. GM names P20C8 for the 3.0 L LM2 Duramax in bulletin 21-NA-149, where a chafed engine harness blows fuses and sets a long list of codes. GM's 2025 DEF tables test the comparable heater C circuit (P20C4) through drain voltage and load current and rate it Type A after 2 s. Open, low and performance faults of heater D are P20C5 to P20C7. In frost the fluid may stay solid, dosing stops and the SCR warning follows.
- What you may notice
- Check engine light and DEF or emission system warning
- DEF dosing unavailable in cold weather, NOx warning or later speed limitation
- Blown fuses and further unrelated codes when a chafed harness is the cause
- Likely causes
- LikelyChafed harness letting the heater D control wire touch a battery-voltage feed
- PossibleMoisture or DEF residue in the heater connector bridging pins
- RareInternal short inside heater D or its integrated pump or line assembly
- RareFaulty heater output stage in the engine or 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: €80 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.