P01A4 Alternative Fuel Rail Pressure Sensor Performance

These codes only apply to vehicles that run on a gaseous alternative fuel – compressed natural gas (CNG) or LPG – either as bi-fuel models alongside petrol (for example GM's 2015–2016 Impala Bi-Fuel or 2013–2016 Silverado/Sierra 2500HD) or as dedicated gas engines. CNG is stored at up to about 250 bar (3600 psi); a regulator reduces it to a few bar for the gas rail and injectors. The gas rail pressure sensor measures the regulated low pressure at the rail – on GM's Impala controlled by the engine module to 68–130 psi (about 4.7–9 bar) – so that injection time can be matched to the actual gas pressure and density. P01A4 is the plausibility fault: the signal is electrically valid but does not match regulator control or operating state – a drifting sensor, often contaminated by oil carried in the gas, or a regulator or shut-off valve behaving unexpectedly. Typical reaction: gas operation is blocked and a bi-fuel vehicle switches to petrol. No manufacturer document naming exactly this code was found; GM's bulletins only state that faults are reported like on petrol models. The description rests on CARB, which treats such vehicles like petrol engines for model years 2004–2018, and on the documented system design.

Likely causes

Symptoms

• Check engine light on
• Switch to petrol or gas operation not selectable
• Hesitation, loss of power or mixture faults in gas mode
• Dedicated gas engines: limp mode or starting problems

Repair estimate

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

Parts involved

Related codes

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