Error Code P0948 is defined as Hydraulic Pump Relay Circuit High. This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to the present. Of course, specifications on the definition, troubleshooting, and repairs vary from one make and model to another. This code also relates to the transmission, including pressure faults and sensor failures.

To generate hydraulic pressure, a vehicle uses a pump. For vehicles with automatic transmission, the channels direct the transmission fluid to the valve body. Both shift solenoid valves and pressure control solenoid valves are located in the valve body. The Transmission Control Module (TCM) manages and activates electronically controlled valves used for shifting the gears. Specifically, the shift solenoid manages multiple components, including the brakes, clutch, and transmission bands. The pressure control solenoid valve (one or more), on the other hand, controls the hydraulic pressure in the transmission.

When this code appears, it means the Transmission Control Module (TCM) has determined an abnormally high voltage (higher than the parameters set by the manufacturer) signal from the hydraulic pump relay circuit.

Common Symptoms

  • Check Engine light activated
  • Rough shifting
  • Vehicle unable to shift
  • Transmission slippage

Possible Causes

  • Dirty or low transmission fluid
  • Faulty wiring or damages related to the electrical connector
  • Faulty hydraulic pump relay
  • Faulty TCM
  • Damaged or defective TCM

How to Check

First, connect the scan tool to the vehicle to retrieve any stored codes. Then, restart and take the vehicle for a test drive to see if the code returns.

Start the diagnosis by checking the quality and level of transmission fluid.

Next, check the transmission pan and look for signs of debris, including clutch debris. This could mean a bigger problem for the transmission.

Then, check the wiring and all electrical components related to the hydraulic pump relay circuit.

Check the harness from the TCM to the transmission and look for signs of damage and wear and tear.

Then, inspect the resistance across different terminals.

If the resistance is at abnormal levels, repeat the resistance test at the transmission connector.

How to Fix

  • Changing the transmission fluid according to the recommendation for the vehicle
  • Transmission fluid flush (if necessary)
  • Repair or replacement of open, shorted, or corroded electrical components or wirings related to the hydraulic pump relay circuit
  • Replacement of damaged or faulty harness from the transmission TCM to the transmission
  • Replacement of faulty hydraulic pump
  • Replacement of faulty TCM
  • Replacement of faulty PCM

Since this code refers to problems in the transmission, it makes the vehicle unsafe to operate. Thus, this error code is considered serious and must be addressed as soon as possible.