P21A9Reductant Level Sensor B Circuit Range/Performance
- 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
- P21A9 flags a level sensor B signal that stays inside its electrical limits but does not fit what else the system knows, such as the states of the other level sensors or the fluid consumed. GM's 2018 Cruze diesel (LH7) stores it in the module running the DEF system, under the name Level Sensor 2 Performance. Engineering bulletin PIE0453 asked dealers for freeze-frame data on ambient and fluid temperature, engine-off time and whether the car was parked outside, because the root cause was still unknown; cold soak is evidently under suspicion. On the 2014 Cruze GM still caught contradictory sensor states under P203B (5 s, Type B). Wiring faults of sensor B go to P21AA/P21AB. Drivers see the MIL and possibly a DEF message.
- What you may notice
- Check engine light on
- DEF level shown wrongly or jumping
- Fault appears mainly after cold nights
- Likely causes
- PossibleFrozen or very cold fluid giving an implausible reading
- PossibleLevel sensor B fouled by crystallised urea or degraded
- RareControl module calibration flagging the sensor too early
- 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.