P2A78 code meaning
Alternative Fuel Hydraulic Pump Pressure Sensor Circuit High
Powertrain system Generic code P2Axx codes Check engine light
What P2A78 means
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.
P2A78 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P2A78
- 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
Common causes of P2A78
- Most likelySensor ground open or corroded at the connector
- PossibleSignal wire shorted to reference or battery voltage
- Less likelyPressure sensor failed with output at full scale
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Alt fuel hyd pump pressure sensorWiring harnessAfcm
What each part of P2A78 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| A | Area: hybrid propulsion. |
| 78 | The exact fault in that area. |
What to do next
- Note P2A78 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P2A78
What does P2A78 mean?
P2A78 means "Alternative Fuel Hydraulic Pump Pressure Sensor Circuit High". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P2A78?
Drive gently and only to a workshop. With P2A78 the car can go into limp mode (reduced power) to protect itself.
Will P2A78 fail an emissions test?
Yes, it can. P2A78 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P2A78 to fix?
Difficulty: hard. A typical repair takes 1 to 3 hours of work. This is a job for a workshop.
Will P2A78 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.