What it means
Intermittent signal from alt fuel rail temp sensor. 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
The alternative fuel rail temperature sensor sends a temperature signal to the control module. P01B8 sets when that signal is intermittent or erratic rather than steady and plausible, often because vibration interrupts a loose connector or damaged harness.
Common causes
Most likely first.
- Connector loose or corroded
- Connector with intermittent contact under vibration
- Harness routed against vibrating component
Symptoms
- Check-engine light may come on intermittently
- Alternative-fuel rail temperature reading may fluctuate or drop out
- Poor running or reduced power may occur
How to diagnose it
- Save freeze-frame data and check whether rail temperature data dropped out or changed abruptly when the code set.
- Watch live rail temperature data while gently moving the sensor connector and accessible harness to try to reproduce the fault.
- Inspect the connector for looseness or corrosion and check whether the harness rubs against or vibrates against nearby components.
- Check the signal wiring and connector continuity while moving the harness, following the manufacturer's wiring information.
- If the wiring remains stable, test the rail temperature sensor for an erratic output using the manufacturer's procedure.
Common fixes
- Secure a loose rail temperature sensor connector and repair corroded pins.
- Reroute and secure the sensor harness away from rubbing or vibration.
- Repair an intermittent section of rail temperature sensor wiring.
- Replace the rail temperature sensor if its output remains erratic with sound wiring.
Can I keep driving?
The rail temperature reading may fluctuate or disappear, potentially affecting alternative-fuel operation and causing poor running or reduced power. Avoid heavy loads if the vehicle runs poorly and arrange diagnosis soon.
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.