What it means
Same as P24CB with the signal shorted to supply. 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 AdBlue/DEF cap is closed. This code sets when its signal rises above the plausible electrical range, commonly because the signal wire is shorted to a supply or pinched against a battery-positive source.
Common causes
Most likely first.
- Signal shorted to supply
- 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
- Check freeze-frame and reductant-system live data to confirm the cap-switch signal is high while the fault is active.
- Inspect the cap-switch harness and connector for pinched insulation, chafing, corrosion, or a wire contacting a power source.
- Move the harness gently while monitoring switch status to see whether its high signal changes with harness position.
- Using the wiring diagram, test the signal circuit for a short to supply and inspect connector terminals for damage or poor contact.
- If the wiring has no short to supply, test the cap switch against the manufacturer’s specification.
Common fixes
- Repair a cap-switch signal wire shorted to a supply or battery-positive source.
- Replace pinched or chafed wiring and secure it away from power sources.
- Repair damaged or corroded connector terminals.
- Replace a faulty cap switch if the signal remains high with sound wiring.
Can I keep driving?
The vehicle may remain driveable, but the reductant or NOx warning can lead to a countdown that eventually prevents restarting. Have the high-signal fault repaired promptly and keep track of any displayed countdown.
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.