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DTC codes / P08xx · Transmission / P0887

P0887TCM Power Relay Control Circuit High

P0887 indicates that the Transmission Control Module (TCM) has detected an abnormally high voltage on the power relay control circuit, which supplies power to the transmission control system. The TCM expects a specific voltage range on this circuit and sets the code when the signal exceeds the upper threshold, often due to a short to voltage or a faulty relay.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • Illumination of the Check Engine Light or Transmission Warning Light
  • Transmission may enter limp mode, limiting gear engagement
  • Harsh or delayed shifting
  • No symptoms besides the warning light in some cases
  • Intermittent loss of power to the transmission control system

Common causes, most likely first

  1. Faulty transmission power relay (stuck closed or internal short)
  2. Short to voltage in the power relay control circuit wiring
  3. Corroded or damaged relay connector terminals
  4. Failed Transmission Control Module (TCM) driver circuit
  5. Poor ground connection for the relay or TCM
  6. Aftermarket accessories improperly wired into the circuit

How to diagnose and fix it

  1. Verify battery voltage and inspect the relay and TCM fuses for corrosion or blown elements
  2. Locate the transmission power relay and swap it with an identical known-good relay to see if the code clears
  3. Disconnect the TCM and relay, then check for voltage on the control circuit with the key on to detect a short to voltage
  4. Inspect the wiring harness between the TCM and relay for chafing, pinched wires, or contamination
  5. Check the relay ground circuit for resistance and repair any excessive resistance or open circuit
  6. If the circuit tests normal, substitute a known-good TCM to rule out an internal driver failure
  7. Clear the code and perform a road test to confirm the repair and ensure no pending codes return

Before you replace the transmission control module

  • A stuck-closed relay is a common cause that is often overlooked because the relay may still click when tested, but its contacts remain welded.
  • The code can be set by a short to voltage from an unrelated circuit that shares the same harness, such as a back-up light or brake switch wire.
  • Many technicians replace the TCM prematurely when the real issue is a corroded connector pin causing high resistance that mimics a high-voltage signal.
  • In some vehicles, the power relay is integrated into the TCM or a power distribution module, making diagnosis more complex and requiring module replacement.
  • Always check for aftermarket wiring or accessories that may have been tapped into the relay control circuit, as these can introduce voltage spikes.

Can I keep driving with P0887?

If the transmission is in limp mode or shifting erratically, avoid highway driving and seek service immediately; if only the warning light is on and the vehicle drives normally, you can drive cautiously to a repair shop but avoid heavy loads or long trips.

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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