P2A77

Code

Generic P — Powertrain

Alternative Fuel Hydraulic Pump Pressure Sensor Circuit Low

Brand: Generic

Type: P — Powertrain

Views: UK: 26 EN: 26 RU: 19

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Page language: EN

Causes

Short to ground in the sensor signal circuit
Open or corroded connector or poor pin contact
Damaged or chafed wiring harness
Failed or out-of-spec hydraulic pump pressure sensor
Loss of sensor reference voltage or poor ground
Faulty pump or mechanical pressure loss producing a very low sensor output

Symptoms

MIL/Check Engine Light illuminated
Alternative-fuel system may go into limp or fail-safe mode
Reduced or altered engine performance on alternative fuel
Inability to switch to or maintain alternative fuel operation
Low or no pressure reading on any dash/scan-tool pressure value
Occasional drivability complaints, stalling or hesitation

What to check

Scan for stored and pending codes and view freeze-frame and live data for the pressure sensor reading
Visually inspect the sensor connector and harness for damage, corrosion, rubbing or pin damage
Backprobe the sensor connector: verify reference voltage (typically ~5 V) and ground with key ON, engine OFF
Measure sensor signal voltage at idle and during pump operation; compare to expected range
Check continuity between the sensor signal pin and the ECU; check for short to ground
Inspect/verify pump operation and system pressure with a mechanical pressure gauge if available

Signal parameters

Reference voltage: typically ~5.0 V (key ON) — verify manufacturer's spec
Signal voltage range (typical): ~0.5 V at zero pressure up to ~4.5 V at maximum pressure (varies by sensor)
Low-circuit threshold: signal significantly below expected idle value or near 0.0–0.2 V indicates low/shorted circuit
Expected circuit continuity: low ohms between sensor ground and vehicle ground; signal circuit should not show continuity to ground when harness disconnected (unless part of design)

Diagnostic algorithm

Safety: Park vehicle, set parking brake, disconnect negative battery if required for repairs.
Scan: Retrieve all codes, freeze frame and live data. Note sensor voltage and related parameters (pump command, system pressure).
Visual: Inspect sensor connector, pins and wiring for corrosion, damage or signs of arcing. Repair obvious damage.
Reference/Ground check: With connector connected, backprobe reference and ground pins with key ON (engine OFF). Confirm proper reference voltage (~5 V) and good ground.
Signal check: Backprobe the sensor signal with key ON and while cranking/engine running if safe. Compare voltage to expected values. If signal is very low or near 0 V, suspect a short to ground or failed sensor.
Isolation: Disconnect the pressure sensor. With harness disconnected, measure resistance between the signal pin and ground to check for short to ground. If short present, trace/repair wiring to find short.
Continuity: Check continuity from the sensor connector to the engine control module connector for open circuits or high resistance.
Sensor swap/test: If wiring and reference/ground are good, replace sensor or try a known-good sensor and re-test.
Mechanical verification: If sensor and wiring test good but pressure reads low, check actual pump output and system pressure with a mechanical gauge; repair or replace pump or system components if pressure is low.
Clear codes and road test: After repair, clear codes, verify live data and perform drive cycle to confirm code does not return.
If fault persists after sensor and wiring verified, consider module diagnostics or consult factory wiring diagrams and service information.