Powertrain, generic (extended)

P21A9 Reductant Level Sensor B Circuit Range/Performance

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

Frozen or very cold fluid giving an implausible reading
Level sensor B fouled by crystallised urea or degraded
Control module calibration flagging the sensor too early

Symptoms

  • Check engine light on
  • DEF level shown wrongly or jumping
  • Fault appears mainly after cold nights

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€0–1,200
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant level sensor

Related codes

P21A8P21AAP21ABP203BP21AEP206B

References: static.nhtsa.gov, gsi.ext.gm.com, law.cornell.edu.