P20C3Reductant Heater C 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
- A low C circuit points to heater C being grounded somewhere or drawing excessive current. GM's 2025 DEF module treats heater 3, its second tank heater, exactly like heaters 1 and 2: a low-band voltage with the output off, or drain voltage and current limits with it on, Type A once 2 s have elapsed. On the 2014 LML Duramax (6.6 L) the glow plug control module reports heater currents above 25 A within about 1 s (Type B). The best-documented case is GM bulletin 22-NA-112: on 2020–2022 Silverado and Sierra with the 3.0 L LM2 or 6.6 L L5P, P20BB and P20C3 could set without any defect, and a recalibration of the DEF control module changed the test. Deutz maps the code to its return line heater (FMI 4). On those trucks software comes first; elsewhere look for a chafed harness.
- What you may notice
- Malfunction indicator lamp, possibly with no real fault present
- Driver message that speed will be reduced without service
- Unheated tank or line section in frost
- Likely causes
- LikelyOutdated DEF control module calibration on 2020–2022 GM diesel pickups (22-NA-112)
- PossibleHeater C wire grounded through chafing
- RareShorted heater C element drawing overcurrent
- 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: €0 – €1,200
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.