What it means
The control module detects a high signal in the reductant level sensor C circuit. The code alone does not confirm an open circuit or a short to battery; 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 reports DEF/AdBlue level information to the control module. P21B0 sets when the signal in sensor C's circuit is higher than expected; the code alone cannot tell whether an open signal line or another circuit condition caused it.
Common causes
Most likely first.
- Open signal line
- Connector pins corroded - high contact resistance
- Supply wire pinched against B+ source
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
- Review freeze-frame data and compare sensor C's reported level with the actual DEF/AdBlue level and other available readings.
- Inspect sensor C's connector and harness for corrosion, loose pins, damaged insulation, or a wire pinched against a battery-positive supply.
- Check for poor contact at the sensor C connector pins, since corrosion or resistance can distort the signal.
- Use the wiring diagram to measure sensor C's signal and supply circuits under the conditions that set the code, comparing readings with the manufacturer's specification.
- If the wiring checks out, test sensor C and its power or ground connections as applicable, then verify the live level reading.
Common fixes
- Clean and secure corroded or loose sensor C connector pins.
- Repair an open or damaged sensor C signal wire.
- Repair wiring pinched against a battery-positive supply.
- Replace sensor C if it fails testing after circuit faults are ruled out.
Can I keep driving?
A short drive is usually possible if the vehicle runs normally, but check the DEF/AdBlue level and take any countdown or restart warning seriously. If the control module cannot trust sensor C's reading, it may issue an incorrect level warning or eventually impose a no-start restriction.
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.