P21C6Reductant Level Sensor B Stuck
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- P21C6 applies only to DEF tanks with a second level measurement, labelled B. It is stored when that second reading is electrically fine but stays frozen at one value although the module expects it to change. No manufacturer documents P21C6 publicly. GM's 2025 diesel tables show how such a check works for sensor 1 (P21C5): during driving with noticeable acceleration and liquid fluid, a level change of less than 0.3 mm per second over roughly 200 s is judged as stuck. Older GM tanks with three discrete level contacts would equate B with contact 2, but GM covers that one only with circuit codes P21AA and P21AB. CARB demands rationality checks for such inputs. Wiring faults of sensor B are P21A8 to P21AC; an untrustworthy level can trigger DEF warnings.
- What you may notice
- Check engine light on
- DEF gauge implausible or not moving
- DEF warning in the instrument cluster
- Likely causes
- LikelySecond level sensing element in the tank unit faulty
- PossibleFloat or contact held by crystals or debris
- RareCalibration of the level evaluation needs a software update
- 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.