What it means
The control module has detected a fault in the reductant level sensor C circuit. The code does not specify the circuit failure or sensor arrangement.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant level sensor C provides DEF/AdBlue level information to the control module. P21AD indicates a fault in sensor C's circuit, but does not specify whether the problem is the sensor, its wiring, connector, power, or ground.
Common causes
Most likely first.
- Failed level sensor C
- Open or shorted sensor wiring
- Loose or corroded connector
- Poor sensor ground or power supply
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
How to diagnose it
- Record freeze-frame data and compare sensor C's reported level with the actual DEF/AdBlue level and other available level readings.
- Inspect sensor C's connector and harness for loose or corroded pins, damaged insulation, or signs of fluid intrusion.
- Check sensor C's power and ground connections for secure contact and measure them against the manufacturer's specification.
- Using the vehicle wiring diagram, test sensor C's signal wire for opens or shorts and compare the signal with the manufacturer's specification.
- If the circuit checks out, test sensor C itself and confirm its reported level changes as expected.
Common fixes
- Clean and secure sensor C's loose or corroded connector.
- Repair an open or shorted section of sensor C wiring.
- Restore a poor sensor C power or ground connection.
- Replace sensor C if it fails testing with a sound circuit.
Can I keep driving?
A short drive is usually possible if the vehicle behaves normally, but check the DEF/AdBlue level and take any countdown or restart warning seriously. A faulty sensor C circuit can make level information unreliable, trigger a no-start restriction, or cause an emissions inspection failure.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.