Powertrain, generic (extended)

P2A78 Alternative Fuel Hydraulic Pump Pressure Sensor Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P2A78 indicates that the hydraulic pump pressure sensor reports a voltage above its plausible maximum, as happens when the signal line touches the reference or battery voltage or the sensor ground is open. A false high reading is critical for the pump cycle, because rising oil pressure is the usual cue that a stroke has ended: Caterpillar's patent US9228574B2 names this as the conventional way to reverse the drive. With a pinned-high signal the controller may reverse too early or halt the pump, and LNG delivery collapses. Westport's design also uses this sensor to spot abnormal conditions, so an over-range value cannot be trusted. No manufacturer publishes P2A78. Inspect the sensor ground and look for chafed wires.

Likely causes

Sensor ground open or corroded at the connector
Signal wire shorted to reference or battery voltage
Pressure sensor failed with output at full scale

Symptoms

  • Warning lamp and very short or no pump strokes
  • Falling gas pressure and loss of power
  • Engine switches to reduced power or stops on gas

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–900
Time1–3 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

P2A75P2A76P2A77P2A79P2A7D

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