P01A8Alternative Fuel Rail/System Pressure - Too Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- 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
- Likely causes
- LikelyGas supply nearly empty, tank pressure close to the regulator minimum
- PossibleHigh- or low-pressure shut-off valve not opening fully (coil, control, manual valve closed)
- PossibleHigh-pressure filter or low-pressure filter/oil separator clogged
- PossiblePressure regulator faulty, sticking or iced (coolant heating disturbed)
- RareLeak on the low-pressure side
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €100 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.