P21ACReductant Level Sensor B Circuit Intermittent/Erratic
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- P21AC reports that the level sensor B signal drops out or jumps for short moments instead of failing steadily. No manufacturer documents criteria for this code publicly. GM's Duramax and Cruze diesel tables cover sensor 2 only with the steady faults P21AA and P21AB, both requiring an error lasting over 3 s, so an intermittent code points to contact losses shorter than that: a terminal that has lost its grip in the tank connector, a harness section that moves with the tank, or corrosion where fluid has crept in. GM's bulletin PIP4887B on related level sensor codes stresses checking terminal tension even when codes are only stored in history. CARB requires input circuits to be watched for such faults (13 CCR 1968.2 (f)(15.2.1)).
- What you may notice
- DEF warning that comes and goes
- Jumping DEF level display
- Check engine light, sometimes only stored code
- Likely causes
- LikelyLoose terminal in the tank connector
- PossibleHarness flexing or rubbing near the tank mounting
- PossibleCorrosion from fluid creeping into 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: €60 – €1,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.