Hydraulic Pump Relay Circuit High
Diagnostic trouble code P0948 indicates that the powertrain control module (PCM) or transmission control module (TCM) has detected an abnormally high voltage signal in the hydraulic pump relay control circuit. This typically means there is a short to voltage or an open circuit preventing the relay from operating the transmission hydraulic pump properly.
This code is considered high severity because a failing hydraulic pump circuit can lead to a loss of transmission fluid pressure. This can cause severe drivability issues, transmission slipping, or complete loss of motive power, potentially leaving you stranded.
Common symptoms
- Illuminated Check Engine Light or Transmission Warning Light
- Transmission slipping or failing to engage gears
- Harsh or erratic shifting
- Vehicle enters limp mode (reduced power)
- Whining noise from the transmission area due to pump starvation or failure
Likely causes
- Faulty hydraulic pump relay
- Wiring harness issue (short to power or open circuit) in the relay control circuit
- Corroded, damaged, or loose electrical connectors
- Blown fuse or fusible link in the hydraulic pump circuit
- Faulty transmission hydraulic pump
- Internal failure in the PCM or TCM (rare)
What to check before replacing parts
Check the transmission hydraulic pump relay fuse.
Look for: A blown fuse indicates a short circuit or an overloaded pump motor. If blown, replace it and see if it blows again immediately.
Swap or test the hydraulic pump relay.
Look for: Swap the relay with a known good identical relay (like the horn relay). If the pump operates and the code clears, the original relay is faulty.
Visually inspect the wiring and connectors for the hydraulic pump relay circuit.
Look for: Look for chafed wires, melted insulation, or corroded pins that could cause a short to voltage or an open circuit.
Measure voltage at the relay control circuit using a digital multimeter.
Look for: With the key on, check for proper command voltage from the TCM/PCM. A constant high voltage when the pump should be off indicates a short to power.
Test the hydraulic pump motor circuit for continuity and resistance.
Look for: Measure the resistance of the pump motor. An out-of-spec reading (too high or infinite) indicates a failing or failed pump motor.
This page explains the generic P0948 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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