Water in Fuel Lamp Circuit Malfunction
The P1139 code indicates a malfunction in the water-in-fuel (WIF) lamp circuit, typically found in diesel vehicles. The powertrain control module (PCM) detects an open or short circuit in the wiring or a faulty sensor that monitors water presence in the fuel filter. This can lead to the warning lamp failing to illuminate when water is detected, or staying on continuously, potentially causing fuel system damage if water accumulation goes unnoticed.
1. Symptoms You Will Notice
- Water-in-fuel warning lamp stays on or off
- Engine may run rough or have reduced performance
- Fuel filter may need frequent draining
- Check Engine Light illuminated
2. Most Common Causes
- Degraded or slow-responding front oxygen sensor (HO2S Bank 1 Sensor 1)
- Sensor contamination from oil, coolant, or fuel additives
- Vacuum leak in the intake system affecting air-fuel mixture
- Wiring or connector issues at the sensor (corrosion, short, or open circuit)
- Clogged or uneven-flow fuel injectors
- Faulty fuel pump or incorrect fuel pressure
- Exhaust leak upstream of the HO2S sensor
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm P1139 and check for related codes.
Step 2: Visually inspect the water-in-fuel sensor and wiring for damage or corrosion.
Step 3: Test the sensor resistance and compare to manufacturer specifications.
Step 4: Check for continuity and shorts in the circuit between sensor and PCM.
Step 5: If wiring is intact, replace the water-in-fuel sensor.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended because water in fuel can damage the injection system. Have it diagnosed promptly.
How much does it cost to fix?▼
Typically $50-$150 for parts and labor, depending on sensor location and labor rates.
5. Commonly Affected Makes
This is a manufacturer-specific code. The exact manufacturer is not confirmed in our current database.