What it means
Same as P248B on heater B. 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
Reductant heater B helps keep DEF/AdBlue from freezing, and its sense circuit reports information the controller uses to monitor heater operation. This code sets when the sense signal is above the plausible range, commonly because the signal wire is shorted to a supply or a connector is making poor contact.
Common causes
Most likely first.
- Sense line 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
- Save freeze-frame data and inspect live data for reductant heater B's sense signal, noting whether it remains high or changes with connector movement.
- Inspect heater B's connector and harness for moisture, corrosion, pushed-back pins, or wiring pinched against a power source.
- Check the sense wire for an unintended short to supply, following the vehicle's wiring diagram and safe circuit-isolation procedure.
- Check terminal fit and continuity between heater B and the controller, including a gentle harness movement to reveal an intermittent contact.
- Compare circuit readings with the manufacturer's specifications and test the heater or controller only after the wiring and connector are checked.
Common fixes
- Repair a sense wire shorted to a supply source.
- Replace damaged or pinched wiring in the heater B sense circuit.
- Clean or replace corroded or loose connector terminals and repair the seal.
- Replace reductant heater B if its sense circuit fails testing.
Can I keep driving?
The vehicle may remain drivable, but a faulty heater signal can compromise DEF thawing and SCR emissions control, especially in cold conditions. Watch for DEF warnings or a no-start countdown and have the fault repaired promptly.
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.