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DTC codes / P09xx · Transmission / P0948

P0948Hydraulic Pump Relay Circuit High

P0948 indicates that the transmission control module (TCM) has detected an abnormally high voltage on the hydraulic pump relay control circuit, which typically points to an open circuit, a short to power, or a failed relay coil. The TCM expects to see a specific voltage range when commanding the relay on or off, and a reading above the threshold triggers this code.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • Illumination of the malfunction indicator lamp (MIL) on the dashboard
  • Often no symptoms besides the warning light, especially if the pump is not currently needed
  • Harsh or delayed transmission shifts due to reduced hydraulic pressure
  • Transmission may enter limp mode or fail to engage certain gears
  • Whining or unusual noises from the transmission pump area if the relay is stuck on
  • In some cases, the vehicle may stall or hesitate during acceleration

Common causes, most likely first

  1. Faulty hydraulic pump relay with an open coil or high resistance
  2. Damaged or corroded wiring harness between the TCM and the relay
  3. Poor electrical connection at the relay socket or TCM connector
  4. Blown fuse or faulty fuse box connection supplying power to the relay circuit
  5. Failed transmission control module (TCM) driver circuit
  6. Short to voltage on the relay control wire due to chafed insulation

How to diagnose and fix it

  1. Inspect the hydraulic pump relay and swap it with a known-good relay of the same type to see if the code clears
  2. Check all fuses related to the transmission control system and replace any that are blown
  3. Disconnect the TCM and relay connectors, then inspect for corrosion, bent pins, or loose terminals and clean or repair as needed
  4. Measure resistance on the relay control wire between the TCM and relay to check for an open circuit or excessive resistance
  5. Test for short to voltage on the relay control wire with the key on and TCM disconnected
  6. Verify power and ground at the relay socket with the ignition on and the relay removed
  7. If all wiring and relays test good, diagnose the TCM driver circuit or replace the TCM as a last resort

Before you replace the transmission control module

  • A common oversight is assuming the relay itself is at fault when the real problem is a corroded connector at the TCM, which can mimic a high-voltage condition on the control circuit.
  • The code can set intermittently due to vibration or temperature changes, so a wiggle test of the harness while monitoring live data often reveals the fault.
  • Some vehicles use a solid-state relay integrated into the TCM, meaning the 'relay' cannot be replaced separately and the entire TCM may need replacement if the driver circuit fails.
  • A weak battery or charging system issue can cause voltage spikes that trigger this code, so always verify battery health and ground connections before condemning the relay or TCM.
  • P0948 is often stored alongside other transmission codes like P0946 or P0947, which can help pinpoint whether the issue is a circuit high or low condition.

Can I keep driving with P0948?

You can usually drive the vehicle with this code, but avoid aggressive acceleration or heavy loads because the hydraulic pump may not operate correctly, leading to poor shift quality or transmission damage over time. Have the fault diagnosed promptly to prevent further complications.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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