Powertrain, generic · Fuel and air metering

P01B4 Alternative Fuel Rail Temperature Sensor Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

Connector or wiring of the rail temperature sensor open, corroded or chafed
Sensor faulty
Gas module, CAN link or engine module

Symptoms

  • Check engine light on
  • Gas operation blocked or switch to petrol
  • Mixture errors or stumbling in gas mode

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€100–450
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

alternative fuel rail temperature sensorgas fuel railsensor wiring

Related codes

P01B5P01B6P01B7P01B8P01A3

References: law.cornell.edu, patents.google.com, static.nhtsa.gov, static.nhtsa.gov.