What it means
The alternative fuel hydraulic pump pressure sensor circuit signal is lower than expected.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
- The engine or gearbox may go into limp mode
How it works
The alternative-fuel hydraulic pump pressure sensor sends a pressure signal to the control module, which monitors the circuit for a plausible level. P2A77 sets when that signal is lower than expected, such as from a signal wire short to ground, an internally shorted sensor, or poor connector contact.
Common causes
Most likely first.
- Signal wire shorted to ground
- Internal sensor short
- Connector pins corroded - high contact resistance
Symptoms
- Alternative-fuel system warning light, if equipped
- Reduced power or poor acceleration
- Hard starting or stalling
How to diagnose it
- Capture freeze-frame data and check live pressure data to confirm the signal is low during the fault.
- Inspect the sensor connector and harness for corroded terminals, damaged insulation, or a signal wire contacting ground.
- Check connector pin fit and corrosion because high contact resistance can pull down the sensor signal.
- Test the signal wire for a short to ground and check the sensor for an internal short using the wiring diagram and manufacturer's procedure.
- If wiring and sensor checks pass, verify the sensor supply, ground, and module input according to the manufacturer's specification.
Common fixes
- Clean and secure corroded or loose pressure sensor connector terminals.
- Repair a signal wire shorted to ground or damaged harness section.
- Replace the pressure sensor if it has an internal short confirmed by testing.
- Restore a confirmed sensor supply or ground fault.
Can I keep driving?
Avoid hard driving if power is reduced or the engine hesitates, since the module may limit alternative-fuel operation and the engine could stall. Use another supported fuel mode if it operates normally, and stop if the vehicle cannot maintain safe power.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.