P01B8
Alternative Fuel Rail Temperature Sensor Circuit Intermittent/Erratic.
- Generic
- Powertrain
- Generic code
- Low severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 1
- AreaFuel and air metering
- B8
- FaultThe specific fault in that area
What it means
P01B8 is a generic code meaning the engine control unit has seen an intermittent or erratic signal from the alternative fuel rail temperature sensor circuit. This sensor is typically fitted on vehicles running a second fuel such as LPG, CNG or similar. The control unit saw the signal jump, drop out or change implausibly quickly compared with what a real temperature change would allow. Which sensor and fuel system are involved depends on the vehicle and engine.
Symptoms
- Engine warning light on
- Poor starting or extended cranking, especially on the alternative fuel
- Rough running or hesitation when using the alternative fuel
- Car switching back to petrol or the main fuel unexpectedly
- Reduced power or poor fuel economy
- Fault appearing and disappearing
Possible causes
- Loose, corroded or damaged connector at the alternative fuel rail temperature sensorlikely
- Chafed, broken or intermittently open or shorted wiring between the sensor and the control unitlikely
- Failing alternative fuel rail temperature sensor with an erratic outputlikely
- Poor sensor earth or reference voltage supply faultpossible
- Water ingress or heat damage in the harness or connector near the fuel railpossible
- Fault within the engine or alternative fuel control unitrare
How to diagnose
- Read all stored and pending codes and note the freeze frame data, including engine temperature and fuel mode when the fault set.
- Check for related codes covering the same sensor, the 5 volt reference, or the alternative fuel system, and deal with those first.
- Inspect the sensor, connector and harness for damage, corrosion, moisture, heat damage and rubbing, and check that the connector locks properly.
- With the engine running on the alternative fuel, watch the live fuel rail temperature reading and gently wiggle the harness and connector to see if the value jumps or drops out.
- Compare the sensor reading with the other temperature readings on a cold engine; they should be broadly similar.
- Check the sensor's resistance against the manufacturer's specification at a known temperature, as the values vary by model.
- Test the sensor supply, signal and earth wiring for continuity, shorts and voltage drop, following the wiring diagram for the vehicle.
- Clear the code and road test on the alternative fuel to confirm the repair and check it does not return.
How to fix
- Clean and refit the connector, or repair or replace damaged terminals.
- Repair or replace the damaged or intermittent wiring section, and secure it away from heat and abrasion.
- Replace the alternative fuel rail temperature sensor if its output is erratic or out of specification.
- Restore a poor earth or supply connection found during testing.
- Where all wiring and the sensor test good, investigate the control unit following the manufacturer's procedure, including whether an updated calibration applies to the model.
Can I keep driving?
The car can usually keep being driven, often on the main fuel if it switches over, but book it in soon. Avoid long journeys if it is running poorly or the warning light is flashing.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.