P2A77: Alternative Fuel Hydraulic Pump Pressure Sensor Circuit Low

P2A77 is set when the signal from the hydraulic pump pressure sensor drops below the range a working sensor can deliver, typically because the signal wire or the 5 V reference is shorted to ground or the reference feed is open. On hydraulically driven LNG pumps this matters more than a mere missing value: Westport's patent US8726785B2 treats a peak hydraulic pressure that repeatedly stays low as the sign of an empty vessel, so a reading stuck near zero can mimic an exhausted tank and trigger needless tank switching or a fuel shutdown. No manufacturer publishes P2A77 or its voltage limits. Check the sensor connector, the reference voltage at the plug and the harness along the frame. P2A78 is the high counterpart.

High severity. Can I keep driving?If you smell gas, do not keep driving or start the engine: move away and call for assistance. With no smell, drive moderately to an authorized shop.

Symptoms

Warning lamp with false empty-tank indication
Unexpected switching between LNG tanks
Gas supply stopped or reduced engine power

Most likely causes

Signal wire chafed through and shorted to ground
5 V reference open or pulled to ground
Pressure sensor failed internally

What to check, in order

Start with the simplest. You can tick off each step.

If you smell gas, shut off the supply, ventilate and do not start the engine
With the connector unplugged, check whether the signal wire is making contact with ground.
Read the codes with a compatible scanner
Check the connectors and wiring of the component indicated
Leave any work on the gas circuit to an authorized shop

Good to know

Repair difficulty: hard
It can trigger limp mode.
It affects emissions.

What each character of P2A77 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
A: Hybrid or air and fuel
77: Specific fault in that system

Frequently asked questions

Can I keep driving with code P2A77?

If you smell gas, do not keep driving or start the engine: move away and call for assistance. With no smell, drive moderately to an authorized shop.

What causes code P2A77?

The most common causes are: signal wire chafed through and shorted to ground; 5 V reference open or pulled to ground; pressure sensor failed internally.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.