What it means
Same as P24CB with intermittent or erratic signal. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant tank cap switch reports whether the DEF/AdBlue tank cap is in the expected position. The control module monitors the switch signal, and this code sets when that signal cuts in and out or changes erratically instead of staying consistent.
Common causes
Most likely first.
- Intermittent contact at connector
- 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
- Check freeze-frame data and live cap-switch status to see whether the signal changes while the cap is stationary.
- Inspect the cap switch and its connector for moisture, corrosion, loose fit, or pushed-back pins.
- Trace the cap-switch harness for chafing or contact with vibrating parts, especially where it bends or is clipped.
- With live data displayed, gently move the harness and connector to see whether the switch reading drops out.
- Test continuity and for unwanted shorts in the cap-switch circuit while moving the harness, then compare results with the manufacturer's specification.
Common fixes
- Clean and secure the cap-switch connector.
- Repair chafed or damaged wiring and secure the harness away from vibrating parts.
- Repair loose or pushed-back connector terminals.
- Replace the faulty cap switch if its signal remains intermittent with sound wiring.
Can I keep driving?
The vehicle may remain drivable, but the DEF/AdBlue warning or a no-start countdown can appear if the fault persists. Avoid ignoring a countdown, since the vehicle may eventually prevent restarting.
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.