P2A78: Alternative Fuel Hydraulic Pump Pressure Sensor Circuit High
What to do
The car will fall back to petrol. Reduced power on gas under sustained load, especially in hot weather, is the symptom that precedes this — if you noticed that before the light came on, say so.
What this code means
The circuit for the hydraulic pump's own pressure sensor is faulty.
A hydraulic fuel pump on a gas system means this is a liquid injection installation, which is a different and more sophisticated thing from the vapour systems the rest of this range describes.
Ordinary LPG conversions vaporise the fuel and inject it as a gas into the inlet manifold. That works, and it costs a little power — gas displaces air, so the engine breathes less of it. Liquid injection keeps the fuel liquid all the way to the injector and sprays it into the port, which recovers that power and actually cools the incoming charge as it flashes to vapour.
The price is that liquid LPG must be kept above its boiling point pressure the whole way, or it vaporises in the line and the injectors deliver nothing. That is what the hydraulic pump is for — it raises tank pressure by several bar so the fuel stays liquid regardless of temperature.
And it is why this hardware has its own pressure sensors and its own select valves. The system has to know it is maintaining that margin continuously, because losing it does not degrade fuelling gradually — the fuel flashes to vapour and delivery collapses at once, usually under sustained load on a hot day, which is exactly when nobody wants it.
These are less common installations, mostly factory or high-end aftermarket, and they need an installer familiar with liquid systems specifically. A workshop used to vapour conversions may not recognise the hardware.
This sensor watches what the pump is producing, where P2A7A watches the pressure in the system it feeds. Both exist so a pump that is working can be told apart from a system that is holding pressure — and losing this one means the car cannot verify the pump is doing anything.
At or above the upper limit.
Likely causes
Start here
- Faulty alternative fuel pressure sensor · specialist
- Corroded or loose connector · specialist
- Open circuit in sensor wiring · specialist
Less common
- Faulty alternative fuel hydraulic pump · specialist
- Damaged wiring harness · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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