P21AA code meaning
Reductant Level Sensor B Circuit Low
Powertrain system Generic code P21xx codes Check engine light
What P21AA means
P21AA means the second reductant level sensor reads too low a voltage, which GM takes as a short to ground. GM documents it for the 6.6L Duramax LML and the 2014 Cruze 2.0 diesel: the tank level electronics apply a test pulse, and if sensor 2 then shows less than 0.17 V for more than 3 s with over 8 V supply, the ECM receives an error status over CAN. The LML treats it as Type A (MIL on the first trip), the Cruze as Type B (two trips). GM bulletin 12-06-04-002C lists P21AA among the DEF codes that must run and pass before the warning tests can clear the driver messages. Look for chafed or wet harness sections first; as the sensors belong to the tank assembly, a failed element usually means a new tank unit.
P21AA 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 P21AA
- Check engine light on
- DEF warning message that stays on
- DEF range or level display wrong
Common causes of P21AA
- Most likelySensor 2 signal wire chafed through to ground
- PossibleFluid or water in the tank connector
- PossibleInternal short in level sensor B (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 P21AA 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. |
| AA | The exact fault in that area. |
What to do next
- Note P21AA 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 P21AA
What does P21AA mean?
P21AA means "Reductant Level Sensor B Circuit Low". P21AA means the second reductant level sensor reads too low a voltage, which GM takes as a short to ground. It is a generic code, so it means the same on every make.
Is it safe to drive with P21AA?
Drive gently and only to a workshop. With P21AA the car can go into limp mode (reduced power) to protect itself.
Will P21AA fail an emissions test?
Yes, it can. P21AA 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 P21AA to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will P21AA clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.