P21AF code meaning
Reductant Level Sensor C Circuit Low
Powertrain system Generic code P21xx codes Check engine light
What P21AF means
P21AF is set when level sensor C, called level sensor 3 by GM, reads below 0.17 V right after the tank electronics apply their test pulse, which GM interprets as a short to ground. Supply must be above 8 V and the error must persist for more than 3 s. The 2014 Cruze diesel stores it as Type B, so the MIL lights after the second failing trip; on the 6.6L LML Duramax it appears in the same form, and GM bulletin 12-06-04-002C places it in the first group of DEF codes to be repaired before the warning tests can run again. When P203C or P21AA are present as well, several sensors reading low at once point to a common wiring or connector problem rather than to sensor 3 alone.
P21AF at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P21AF
- Check engine light after two trips
- DEF warning message
- Implausible DEF level shown
Common causes of P21AF
- Most likelySensor 3 signal wire shorted to ground
- PossibleWater or fluid bridging contacts in the tank connector
- PossibleLevel sensor C shorted inside the tank assembly
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 P21AF 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. |
| AF | The exact fault in that area. |
What to do next
- Note P21AF 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 P21AF
What does P21AF mean?
P21AF means "Reductant Level Sensor C Circuit Low". P21AF is set when level sensor C, called level sensor 3 by GM, reads below 0.17 V right after the tank electronics apply their test pulse, which GM interprets as a short to ground. It is a generic code, so it means the same on every make.
Is it safe to drive with P21AF?
Drive gently and only to a workshop. With P21AF the car can go into limp mode (reduced power) to protect itself.
Will P21AF fail an emissions test?
Yes, it can. P21AF 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 P21AF to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will P21AF clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.