Powertrain, generic (extended)

P2A77 Alternative Fuel Hydraulic Pump Pressure Sensor Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

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

Symptoms

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

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

P2A75P2A76P2A78P2A79P2A7C

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