What it means
The reductant level sensor C circuit signal is intermittent or erratic. The code does not identify whether the cause is the sensor, wiring, or connection.
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. P21B1 indicates that sensor C's circuit signal is intermittent or erratic, so the reported level may fluctuate; the code does not identify whether the sender, wiring, or connection is responsible.
Common causes
Most likely first.
- Corroded sender contact
- Connector with intermittent contact under vibration
- Harness routed against vibrating component
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 scan-tool history for changes in sensor C's reported level around the fault.
- Compare the reported level with the actual DEF/AdBlue level and watch sensor C's live reading for sudden jumps.
- Inspect sensor C's connector for corrosion and confirm the pins are seated and make firm contact.
- Check whether the sensor C harness runs against a vibrating component, then gently move it while monitoring the live reading for dropouts.
- If the signal remains erratic, test sensor C and its circuit connections against the manufacturer's specification.
Common fixes
- Clean and secure a corroded sensor C sender contact.
- Repair loose or intermittent contact at sensor C's connector.
- Reroute or secure the sensor C harness away from vibrating components.
- Replace sensor C if its signal remains erratic with sound wiring and connections.
Can I keep driving?
If the vehicle runs normally, a short trip is generally reasonable, but verify the DEF/AdBlue level and watch for a countdown or restart warning. Persistent signal dropouts can make the level warning 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.