What it means
Same as P238A with the signal voltage below the plausible range. 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
How it works
On diesel vehicles with a secondary water-in-fuel sensor, sensor B monitors water at its filter-housing stage. P238C sets when the control module sees sensor B’s signal below the plausible range, commonly due to a short to ground, chafed wire, moisture, or a poor terminal connection.
Common causes
Most likely first.
- Signal shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Check-engine light on
- Water-in-fuel warning may be unavailable or inaccurate
How to diagnose it
- Record freeze-frame data and check whether the sensor B signal remains low while the fault is active.
- Locate sensor B at the secondary filter housing using vehicle service information and inspect for moisture, corrosion, pin push-back, or damaged seals.
- Check the sensor B harness for chafing or crushed insulation, then gently move it while watching the live signal for a change.
- Use the wiring diagram to test for a signal short to ground and check signal, supply, and ground circuits against the manufacturer’s specification.
- If the wiring and connections pass, test sensor B using the manufacturer’s procedure.
Common fixes
- Repair a sensor B signal wire shorted to ground or chafed through its insulation.
- Dry and clean the connector, then repair damaged seals that allow water ingress.
- Restore proper pin position and terminal contact at the sensor B connector.
- Replace sensor B if it fails the manufacturer’s test.
Can I keep driving?
Assume the water-in-fuel warning may be unavailable or inaccurate and avoid extended driving until the circuit is repaired. Undetected water can reach and damage the diesel high-pressure pump and injectors.
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.