P20C4Reductant Heater C 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
- P20C4 marks heater C's control circuit as high, meaning it sees battery voltage where none belongs. GM's 2025 tables test heater 3, a second tank heater, with the output switched off against a raised voltage window and set a Type A fault after 2 s of persistence. GM's LML documentation from 2014 (6.6 L) has the glow plug module react to over 3.5 V on the heater pin while the heater should be off (Type B). Deutz lists its return line heater with FMI 3, voltage above normal. No public service bulletin names P20C4, so the main suspects are harness damage near a supply wire and an output stage that stays on. An element that keeps heating with the engine off drains the battery; once the module locks it out, that section freezes. Open and low faults of heater C are P20C1 and P20C3.
- What you may notice
- Check engine light and SCR warning
- Battery drained overnight if the heater stays powered
- Inducement and speed limit if left unrepaired
- Likely causes
- LikelyHeater C control wire in contact with a battery feed
- PossibleHeater driver in the DEF or glow plug module stuck on
- RareDEF crystals or moisture bridging pins in the heater connector
- 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: €60 – €1,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.