Fuel and air

P01B8 — Alternative Fuel Rail Temperature Sensor Circuit Intermittent/Erratic

Severity: medium Drive with care, to a shop

What P01B8 means

The engine control module monitors the signal from the fuel rail temperature sensor used with the alternative fuel system (such as gas or ethanol blends). P01B8 is set when that signal drops out, jumps around or does not match realistic temperature changes. The data helps the module calculate fuel delivery, so an unstable signal reduces accuracy.

Symptoms

  • Check Engine light that comes and goes
  • Fuel rail temperature reading jumping on a scan tool
  • Rough running or hesitation on acceleration
  • Reduced power
  • Hard starting on the alternative fuel
  • Possible increase in fuel consumption

Possible causes

  1. Corroded or loose sensor connector
  2. Intermittent open or short in the wiring harness
  3. Failing fuel rail temperature sensor
  4. Damaged insulation causing a short to ground or voltage
  5. Rarely, a fault in the PCM input circuit

How to diagnose

  1. Read codes and freeze frame data, clear the code and see if it returns
  2. Inspect the sensor connector for a loose lock, corrosion, moisture or bent pins
  3. Watch fuel rail temperature live on a scan tool with a warm engine
  4. Wiggle the harness and connector with the engine running and watch for signal spikes
  5. Measure sensor resistance and compare it with the manufacturer's specification
  6. Test signal and ground wires to the PCM for opens and shorts
  7. Check the reference voltage at the sensor connector
  8. If everything tests good, inspect the PCM connector before suspecting the module

Common fixes

  • Clean the terminals and secure the connector
  • Repair or replace the damaged section of wiring
  • Replace the fuel rail temperature sensor
  • Repair or replace the PCM only after ruling out everything else

Questions and answers

What does P01B8 mean?

The PCM detected an unstable, intermittent signal in the alternative fuel rail temperature sensor circuit. The connector, wiring or sensor itself is the most common culprit.

Can I drive with code P01B8?

Usually yes, but only for a short time and with care. Unreliable data can upset fuel metering, so have it diagnosed soon.

Why does the code come and go?

The fault is intermittent: vibration, moisture or worn wiring breaks the connection from time to time. Wiggling the harness and connector while watching live data helps reveal it.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.