Powertrain, generic (extended)

P21C6 Reductant Level Sensor B Stuck

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

Second level sensing element in the tank unit faulty
Float or contact held by crystals or debris
Calibration of the level evaluation needs a software update

Symptoms

  • Check engine light on
  • DEF gauge implausible or not moving
  • DEF warning in the instrument cluster

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€0–1,200
Time0.3–4 h

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

Parts involved

reductant level sensor

Related codes

P21C5P21A8P21A9P21AA

References: law.cornell.edu, gsi.ext.gm.com, gsi.ext.gm.com.