P01B4: Alternative Fuel Rail Temperature Sensor Circuit

What it means

Generic electrical fault on alternative fuel rail temperature sensor (CNG/LPG/H2 systems).

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 measures the temperature of fuel in the rail, and the control module uses its electrical signal to manage the alternative-fuel system. P01B4 indicates a general electrical fault in this sensor circuit, which may leave the reading inaccurate or unavailable.

Common causes

Most likely first.

1. Temperature sensor failure
2. Open or short in sensor wiring
3. Loose, damaged, or corroded connector

Symptoms

• Check-engine light on
• Alternative-fuel rail temperature reading may be inaccurate or unavailable
• Poor running or reduced power may occur

How to diagnose it

1. Save freeze-frame data and check the alternative-fuel rail temperature reading for an implausible value or missing data.
2. Inspect the rail temperature sensor connector and nearby wiring for looseness, corrosion, heat damage, or broken insulation.
3. Check the sensor signal, supply, and ground circuits for an open or short using the manufacturer's wiring information.
4. Test the rail temperature sensor and compare its output with the manufacturer's specification under the specified conditions.
5. If the electrical checks pass, review live temperature data as the system operates to identify a reading that does not respond as expected.

Common fixes

• Reconnect or clean a loose or corroded rail temperature sensor connector.
• Repair an open, shorted, or damaged section of sensor wiring.
• Replace the alternative fuel rail temperature sensor if it fails the manufacturer's test.

Can I keep driving?

You may have poor running or reduced power if the alternative-fuel system cannot use a reliable rail temperature reading. Avoid demanding driving if the engine runs poorly, and have the fault checked promptly.