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DTC codes / P08xx · Transmission / P0885

P0885TCM Power Relay Control Circuit/Open

P0885 indicates that the transmission control module (TCM) has detected an open circuit or excessive resistance in the power relay control circuit, which supplies power to the TCM itself. The TCM monitors the voltage on its power supply circuit and sets this code when the expected voltage is not present, often due to a faulty relay, blown fuse, or wiring issue.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • The check engine light or transmission warning light illuminates on the dashboard.
  • The transmission may enter limp mode, limiting gear selection to a single gear or neutral.
  • Harsh or delayed shifting may occur as the TCM loses power intermittently.
  • The vehicle may fail to start or stall if the TCM is not powered.
  • Often no symptoms besides the warning light, especially if the fault is intermittent.

Common causes, most likely first

  1. Blown fuse or fusible link in the TCM power circuit.
  2. Defective TCM power relay (open coil or stuck contacts).
  3. Corroded, loose, or damaged wiring or connectors in the relay control circuit.
  4. Poor ground connection for the TCM or relay.
  5. Faulty TCM itself, though less common.
  6. Water intrusion or physical damage to the wiring harness.

How to diagnose and fix it

  1. Check all fuses related to the TCM and transmission system, including underhood and interior fuse boxes.
  2. Inspect the TCM power relay by swapping it with an identical relay from a non-critical circuit and see if the code returns.
  3. Perform a voltage drop test on the power and ground circuits at the TCM connector to identify high resistance.
  4. Visually inspect wiring and connectors for corrosion, chafing, or loose pins, especially near the transmission and underhood areas.
  5. Test the relay control circuit from the TCM to the relay for continuity and short to ground or power.
  6. If all circuits test good, substitute a known-good TCM to confirm if the internal driver has failed.
  7. Clear codes and perform a road test to verify the repair and ensure the code does not reset.

Before you replace the TCM

  • A faulty ignition switch or main power relay can cause this code because the TCM power relay often receives its control signal from the ignition circuit.
  • Corrosion inside the TCM connector pins is a frequent culprit, especially in vehicles with leaks or high humidity, and can mimic a failed TCM.
  • The code may set intermittently due to a relay that tests fine when cold but fails when hot; always test the relay under load or swap it.
  • Some vehicles use a shared power relay for multiple modules, so a failure may also set codes in other systems like the ABS or engine control.
  • A weak battery or charging system issue can cause voltage drops that trigger this code, so always check battery health and connections first.

Can I keep driving with P0885?

If the vehicle is in limp mode or shifting poorly, avoid highway driving and seek repair promptly to prevent transmission damage. If the only symptom is the warning light and the car drives normally, you can drive cautiously to a shop, but avoid heavy loads or aggressive acceleration.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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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