What it means
The transmission control module monitors the electrical control circuit for its power relay. It sets this code when the circuit voltage is above the expected range.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
The TCM power relay supplies the transmission control system, while the TCM monitors the relay's control circuit. P0887 sets when that circuit reads higher than expected, which can point to a wire shorted to battery positive, an open relay coil, or a poor connector contact.
Common causes
Most likely first.
- Control wire shorted to B+
- Open relay coil
- Connector pins corroded - high contact resistance
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Save freeze-frame data and review live relay-command and supply readings to identify when the circuit goes high.
- Inspect the relay connector and control-wire route for chafing against a power feed, corrosion, loose pins, or pin push-back.
- Check the relay coil and control wire for an open circuit using the vehicle's wiring diagram.
- Measure the control-circuit signal with the relay commanded and compare it with the manufacturer's specification.
- If the circuit and relay test correctly, check the TCM control output using the manufacturer's diagnostic procedure.
Common fixes
- Clean and secure corroded or loose connector pins.
- Repair the control wire if it is shorted to battery positive.
- Replace an open-circuit relay coil or faulty power relay.
- Repair the TCM control circuit if testing confirms an internal fault.
Can I keep driving?
Limit driving if shifts are harsh, delayed, or the vehicle enters limp mode, since the transmission may lose normal control. Avoid towing or hard acceleration, and stop if the transmission slips or gear selection becomes unreliable.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.