P21B1Reductant Level Sensor C Circuit Intermittent/Erratic
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- P21B1 marks level sensor C signals that come and go. Public manufacturer criteria do not exist; GM's diesel tables describe sensor 3 only through the steady faults P21AF and P21B0, each filtered over 3 s. A stored P21B1 therefore suggests interruptions too short for those monitors, typical of a connector that works loose under tank vibration, fluid seeping into the terminals, or a harness pinched under a tank strap. Moving the harness while watching the sensor data with a scan tool is the quickest way to reproduce it. If P21AC is present too, the cause probably lies in wiring both sensors share. Drivers mainly notice an unsteady DEF level display or a warning that appears and clears again.
- What you may notice
- Unsteady DEF level display
- DEF warning that appears and clears
- Stored code without current fault
- Likely causes
- LikelyTank connector loosening under vibration
- PossibleFluid seeping into the terminals
- RareHarness pinched under a tank strap
- 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.