P2A78Alternative Fuel Hydraulic Pump Pressure Sensor Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- 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
- Likely causes
- LikelySensor ground open or corroded at the connector
- PossibleSignal wire shorted to reference or battery voltage
- RarePressure sensor failed with output at full scale
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €100 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.