P2A79: Alternative Fuel Hydraulic Pump Pressure Sensor Circuit Intermittent/Erratic

P2A79 records a hydraulic pump pressure signal that jumps or drops out for short moments without staying low or high long enough for P2A77 or P2A78. In a reciprocating LNG pump the controller reads this sensor continuously to find each stroke end, so a single spike can be mistaken for the end of travel and a brief dropout for a missing pressure peak. The pump may then run uneven strokes, and Westport's empty-tank logic, which counts low peaks over several strokes, can be misled. Vibration at the frame-mounted hydraulic unit, moisture in the plug and fractured crimps are typical causes. No manufacturer documents P2A79. Log the signal while flexing the harness; dropouts in an otherwise steady trace point to wiring.

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

Uneven fuel pressure or surging power on gas
Sporadic warning lamp that clears again
Occasional false empty-tank message

Most likely causes

Loose or fractured crimp terminal in the sensor circuit
Moisture or ice in the sensor plug
Harness damaged by vibration at the hydraulic unit

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
Gently move the wiring with the engine running and watch whether the reading changes.
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 P2A79 means

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

Frequently asked questions

Can I keep driving with code P2A79?

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 P2A79?

The most common causes are: loose or fractured crimp terminal in the sensor circuit; moisture or ice in the sensor plug; harness damaged by vibration at the hydraulic unit.

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.