What it means
The reductant level sensor B 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 B sends DEF/AdBlue level information to the control module. P21AC sets when sensor B's circuit signal becomes intermittent or erratic, which can make the reported level fluctuate; the code alone does not distinguish a sender fault from a wiring or connection problem.
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 B's reported level around the time the fault occurred.
- Compare the displayed DEF/AdBlue level with the actual level and watch sensor B's live reading for sudden jumps while the vehicle is stationary.
- Inspect sensor B's connector for corrosion and check that its pins remain seated and make firm contact.
- Check the sensor B harness routing for contact with vibrating parts, then gently move the harness while monitoring the live signal for dropouts.
- If the signal still fluctuates, test sensor B and its circuit connections against the manufacturer's specification.
Common fixes
- Clean and secure a corroded sensor B sender contact.
- Repair a loose connector or intermittent pin contact at sensor B.
- Reroute or secure the sensor B harness away from vibrating components.
- Replace sensor B 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. Continued erratic readings can trigger an inaccurate level warning or a no-start restriction and may prevent an emissions inspection.
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.