P238B: Water in Fuel Sensor B Circuit Range or Performance

What it means

Same as P238A with the signal in plausible range yet not matching the engine model.

At a glance

• Usually lights the check-engine light

How it works

On diesel systems fitted with a secondary water-in-fuel sensor, sensor B monitors water at its filter-housing stage and helps the control module interpret that condition. P238B means the signal is electrically plausible but does not match the module’s expected model, which can result from sensor drift, wiring trouble, or a contaminated connection.

Common causes

Most likely first.

1. Water-in-fuel sensor B fault or drift
2. Contaminated or damaged sensor connector
3. Wiring fault affecting the sensor signal

Symptoms

• Check-engine or water-in-fuel warning light on
• Possible reduced engine power

How to diagnose it

1. Save freeze-frame data and review the sensor B reading and water-in-fuel status with a scan tool.
2. Locate the secondary filter housing and sensor B using vehicle service information, then inspect the connector for corrosion, contamination, or damage.
3. Check the harness for chafing, poor terminal fit, or moisture at the connector seals.
4. Measure sensor B signal, supply, and ground against the manufacturer’s specification and check whether the reading changes plausibly.
5. If the circuit is sound, test sensor B and verify it is correctly installed in the secondary filter housing.

Common fixes

• Clean and secure the sensor B connector.
• Repair damaged wiring, poor terminal contact, or a leaking connector seal.
• Correct sensor installation if it is not seated properly.
• Replace sensor B if its output remains implausible and it fails the manufacturer’s test.

Can I keep driving?

The water-in-fuel warning may be inaccurate, so avoid extended driving until sensor B feedback is restored, particularly if the warning is on. Undetected water can damage the diesel high-pressure pump and injectors.