What it means
The control module detects a low signal in the reductant level sensor C circuit. The code alone does not confirm a short to ground; the cause depends on the circuit design.
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 sends DEF/AdBlue level information to the control module. The module sets P21AF when sensor C's circuit signal is lower than expected; that low reading does not by itself prove a short to ground.
Common causes
Most likely first.
- Short to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
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 any other available level readings.
- Inspect sensor C's harness for chafing through its insulation and check the connector for corrosion, loose pins, or moisture.
- Check sensor C's ground and power connections, as applicable, against the manufacturer's specification.
- Use the wiring diagram to test sensor C's signal circuit for a short to ground or excessive resistance, comparing results with the manufacturer's specification.
- If the wiring and connections are sound, test sensor C's output and replace it only if it remains low under the specified test conditions.
Common fixes
- Clean and secure corroded sensor C connector pins.
- Repair chafed sensor C wiring and protect it from further contact.
- Repair a confirmed short to ground or poor circuit connection.
- Replace sensor C if its output remains below specification with a sound circuit.
Can I keep driving?
If the vehicle runs normally, a short trip is generally reasonable, but check the DEF/AdBlue level and heed any countdown or restart warning. The low reading can leave level information unreliable and may lead to a no-start restriction or emissions-test 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.