What it means
Same as P24CB with the signal shorted to ground. 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 is below the plausible electrical range, typically because the signal wire is shorted to ground or the connector has a poor connection.
Common causes
Most likely first.
- Signal shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
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
- Use freeze-frame and reductant-system live data to confirm the cap-switch signal is low while the fault is present.
- Inspect the cap-switch wiring and connector for chafing, moisture, corrosion, pushed-back pins, or damaged insulation.
- Move the harness gently while watching switch status to check whether the low signal changes with harness position.
- Using the wiring diagram, test the signal circuit for a short to ground and check the connector terminals for poor contact.
- If the circuit and connector pass, test the cap switch against the manufacturer’s specification.
Common fixes
- Repair a cap-switch signal wire shorted to ground.
- Clean or repair corroded or pushed-back connector terminals and restore connector sealing.
- Repair and secure chafed wiring in the reductant tank area.
- Replace a faulty cap switch if the signal remains low with sound wiring.
Can I keep driving?
You can usually drive if there are no other symptoms, but the reductant or NOx warning may lead to a countdown that eventually prevents restarting. Repair the low-signal fault promptly and monitor any AdBlue/DEF 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.