P21C6 code meaning
Reductant Level Sensor B Stuck
Powertrain system Generic code P21xx codes Check engine light
What P21C6 means
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.
P21C6 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.3 to 4 hours
- Clears after repairAfter a drive cycle
Symptoms of P21C6
- Check engine light on
- DEF gauge implausible or not moving
- DEF warning in the instrument cluster
Common causes of P21C6
- Most likelySecond level sensing element in the tank unit faulty
- PossibleFloat or contact held by crystals or debris
- Less likelyCalibration of the level evaluation needs a software update
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Reductant level sensor
What each part of P21C6 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 1 | Area: fuel and air metering. |
| C6 | The exact fault in that area. |
What to do next
- Note P21C6 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P21C6
What does P21C6 mean?
P21C6 means "Reductant Level Sensor B Stuck". P21C6 applies only to DEF tanks with a second level measurement, labelled B. It is a generic code, so it means the same on every make.
Is it safe to drive with P21C6?
Drive gently and only to a workshop. With P21C6 the car can go into limp mode (reduced power) to protect itself.
Will P21C6 fail an emissions test?
Yes, it can. P21C6 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P21C6 to fix?
Difficulty: medium. A typical repair takes 0.3 to 4 hours of work. This is a job for a workshop.
Will P21C6 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.