What it means
The TCM monitors the output side of its power relay; when the sense voltage is below threshold while relay is commanded closed, this code sets.
At a glance
- Usually lights the check-engine light
How it works
The TCM power relay supplies the transmission control system, and the TCM monitors voltage on the relay's output side through a sense input. P0890 sets when the relay is commanded closed but the sensed output remains below the expected level, which can indicate a relay that failed open, an open sense wire, or a poor connection.
Common causes
Most likely first.
- Relay failed open
- Open wiring at sense input
- 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
- Record freeze-frame data and compare live relay command with the output-side sense reading to confirm whether the low reading occurs while the relay should be closed.
- Inspect the relay, its socket, and the sense-input connector for corrosion, heat damage, loose terminals, or pin push-back.
- Check continuity in the sense wire and power path using the vehicle's wiring diagram to locate an open circuit.
- Measure relay output and sense-circuit readings while the relay is commanded and compare them with the manufacturer's specification.
- If the wiring is intact, test relay switching and the TCM sense input using the manufacturer's diagnostic procedure.
Common fixes
- Clean and secure corroded or loose relay and sense-input connections.
- Repair an open sense wire or power feed.
- Replace a relay that fails to close or provide its specified output.
- Repair the TCM sense input circuit if testing confirms an internal fault.
Can I keep driving?
If shifting is normal, drive only as needed and watch for warning lights or changes in gear behavior. Avoid driving if shifts become harsh or delayed or limp mode begins, because the TCM may lose reliable power-relay feedback.
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.