Powertrain, generic (extended)

P21A8 Reductant Level Sensor B Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P21A8 is the general circuit code for the second reductant level sensor, needed only where a DEF tank reports its fill level through more than one sensing element. GM's 6.6L Duramax (LML/LGH) and the 2014 Cruze diesel read three discrete level sensors, numbered 1 to 3 in GM's tables, which corresponds to A to C. GM does not use P21A8 itself but splits the fault into P21AA (below 0.17 V after a test pulse) and P21AB (open load or short to battery). A maker using the umbrella code stores it for any wiring fault it cannot assign to one side. CARB requires such input circuits to be monitored (13 CCR 1968.2 (f)(15.2.1)). Expect a DEF warning message and, if left unrepaired, the usual SCR warning sequence.

Likely causes

Damaged or corroded harness or connector at the DEF tank
Level sensor B in the tank assembly failed
Tank level evaluation electronics faulty

Symptoms

  • Check engine light on
  • DEF system warning message in the cluster
  • DEF level or range display missing or wrong

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–1,200
Time0.5–4 h

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

Parts involved

reductant level sensor

Related codes

P21A9P21AAP21ABP21ACP203AP21AD

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