Powertrain, generic (extended)

P2A79 Alternative Fuel Hydraulic Pump Pressure Sensor Circuit Intermittent/Erratic

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

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

Symptoms

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

Repair estimate

DifficultyHardWorkshop job
Parts and labour€80–700
Time1–4 h

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

Parts involved

alt fuel hyd pump pressure sensorwiring harnessafcm

Related codes

P2A75P2A77P2A78P2A7EP2A70

References: law.cornell.edu, patents.google.com.