Powertrain, generic (extended)

P21C5 Reductant Level Sensor A Stuck

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

P21C5 is a plausibility code: the level sensor A reports valid readings, yet they hardly move when the fluid in the tank should be sloshing. GM spells this out for the ultrasonic sensor in its 2025 diesel tables, where it is sensor 1. While the vehicle is above 5 km/h, accelerating by more than 0.1 m/s², the fluid is liquid and over 5 L remain, a level change below 0.3 mm within one second counts as a failed sample; after about 200 s of such samples the module sets the code as Type B, so the MIL comes on after two trips. Jaguar issued SSM73632 for the 2017 to 2018 F-Pace with P21C5-64 and blamed a level sensor calibration, fixed by new engine software. Electrical faults of the same sensor are P203A to P203E, sensor B stuck is P21C6.

Likely causes

Level sensor calibration in the engine software (update available, e.g. Jaguar)
Ultrasonic level sensor in the tank unit faulty
Crystal deposits or gas bubbles disturbing the echo
Fluid partly frozen or slush around the sensor

Symptoms

  • Check engine light on after two drives
  • DEF gauge stays at the same value while fluid is being used
  • DEF warning messages 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

P21C6P203AP203BP21A9

References: gsi.ext.gm.com, static.nhtsa.gov.