P20BBReductant Heater A 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 reading on the heater A control circuit usually means the output or its wire is grounded, or the element pulls far more current than it should. GM's 2025 DEF module checks heater 1 (tank heater 1) in both states: switched on, a low-side drain voltage above 0.45 V or a load current above 95 A counts; switched off, the high-side voltage sitting in a low band near 0.5–0.6 V. Two seconds of failures give a Type A fault. Ford logs P20BB on circuit 1 above 15 A and lets the glow plug module report it within 250 ms; VW uses a signal voltage under 2.97 V on the 2014 Touareg. On 2020–2022 Silverado and Sierra diesels GM found the test itself setting P20BB falsely and released new DEF module calibrations (22-NA-112). Expect a warning lamp and, later, a speed limit.
- What you may notice
- Malfunction indicator lamp, possibly with a blown heater fuse
- Message that speed will be reduced if the system is not serviced
- No DEF heating, so dosing is delayed in frost
- Likely causes
- LikelyHeater A wire chafed through and touching ground
- PossibleInternal short in the heater element, drawing excessive current
- PossibleDEF control module calibration setting the code falsely (GM 22-NA-112)
- RareMoisture bridging heater pins to ground in the 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: €0 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.