These codes only apply to vehicles with a gaseous alternative fuel – compressed natural gas (CNG) or LPG – as bi-fuel models alongside petrol or as dedicated gas engines; on bi-fuel vehicles a fault usually blocks gas operation and the vehicle runs on petrol. The gas rail temperature sensor, often combined with the rail pressure sensor, measures the gas temperature at the injectors: gas cools strongly when it expands in the regulator, and the injected mass depends on density, i.e. on rail pressure and temperature, so the module corrects injection time with it. A Ford patent describes plausibility checks against engine bay temperature at idle and an expected cold rail at full load with a full tank. Roush propane systems report rail temperature faults as P0181–P0183; no manufacturer document using exactly this code was found. P01B4 is the general circuit fault of this sensor.
High severity. Can I keep driving?If you smell gas, do not keep driving or start the engine: move away and call for assistance. With no smell, drive moderately to an authorized shop.
Symptoms
- Check engine light on
- Gas operation blocked or switch to petrol
- Mixture errors or stumbling in gas mode
Most likely causes
- Connector or wiring of the rail temperature sensor open, corroded or chafed
- Sensor faulty
- Gas module, CAN link or engine module
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It can trigger limp mode.
- It affects emissions.
What each character of P01B4 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 1: Air and fuel
- B4: Specific fault in that system
Frequently asked questions
Can I keep driving with code P01B4?
If you smell gas, do not keep driving or start the engine: move away and call for assistance. With no smell, drive moderately to an authorized shop.
What causes code P01B4?
The most common causes are: connector or wiring of the rail temperature sensor open, corroded or chafed; sensor faulty; gas module, CAN link or engine module.
Related codes
- P01B5Alternative Fuel Rail Temperature Sensor Performance
- P01B6Alternative Fuel Rail Temperature Sensor Circuit Low
- P01B7Alternative Fuel Rail Temperature Sensor Circuit High
- P01B8Alternative Fuel Rail Temperature Sensor Circuit Intermittent/Erratic
- P01A3Alternative Fuel Rail Pressure Sensor Circuit
Sources
- law.cornell.edu: 13 CCR 1968.2
- patents.google.com: US9004050
- static.nhtsa.gov: MC 10118628 9999
- static.nhtsa.gov: MC 10114282 9999
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.