Powertrain, generic · Fuel and air metering

P01A8 Alternative Fuel Rail/System Pressure - Too Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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. P01A8 is not a sensor fault but the gas counterpart of P0087: the measured rail or system pressure stays below target. Typical reasons are a nearly empty tank close to the regulator's minimum inlet pressure, a shut-off valve that does not open fully, clogged high- or low-pressure filters, or a regulator that sticks or ices up when its coolant heating fails. 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

Gas supply nearly empty, tank pressure close to the regulator minimum
High- or low-pressure shut-off valve not opening fully (coil, control, manual valve closed)
High-pressure filter or low-pressure filter/oil separator clogged
Pressure regulator faulty, sticking or iced (coolant heating disturbed)
Leak on the low-pressure side

Symptoms

  • Check engine light on
  • Automatic switch to petrol or gas operation not possible
  • Loss of power or hesitation under load in gas mode
  • Possibly lean codes (P0171/P0174) in gas mode

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–1,500
Time1–4 h

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

Parts involved

gas pressure regulatorgas shutoff valvegas filtergas fuel rail

Related codes

P01A9P01A4P019FP0087P0171

References: law.cornell.edu, static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov.