What it means
The alternative fuel hydraulic pump pressure sensor circuit signal is intermittent or erratic.
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 provides a pressure signal that the control module uses to manage the system. P2A79 sets when that signal changes unpredictably or drops in and out, often because the connector is loose, the ground reference is noisy, or vibration interrupts contact.
Common causes
Most likely first.
- Sensor connector not seated
- Noisy ground reference
- Connector with intermittent contact under vibration
Symptoms
- Alternative-fuel system warning light, if equipped
- Intermittent loss of power
- Hesitation or stalling
How to diagnose it
- Save freeze-frame data and review live hydraulic pressure data to identify when the signal became erratic and whether it coincided with engine load or vibration.
- Check that the pump pressure sensor connector is fully seated and inspect its terminals and wiring for looseness, corrosion, or movement.
- Inspect the sensor ground and ground-reference wiring for damage or poor connections that could make the reading unstable.
- Monitor the sensor signal with a scan tool while gently moving the harness and connector to locate a vibration-sensitive interruption.
- If the connector and wiring remain sound, test the sensor and its ground reference against the manufacturer's specification.
Common fixes
- Reseat and secure the alternative-fuel hydraulic pressure sensor connector.
- Clean or repair loose, corroded, or vibration-sensitive connector terminals.
- Repair a damaged ground-reference wire or poor ground connection.
- Replace the pressure sensor if its signal remains erratic with a verified connector and ground.
Can I keep driving?
Because the fault can cause intermittent power loss, hesitation, or stalling, avoid high-speed trips and situations where a stall would be dangerous. If the engine begins cutting out, pull over safely and have the fault addressed.
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.