TCM Power Relay Sense Circuit Range/Performance
Your transmission control computer can't properly sense its power relay, like a light switch that won't reliably tell if it's on or off. This causes the TCM to lose confidence in its own power supply stability.
The TCM monitors the voltage feedback from its power relay sense circuit to confirm stable power delivery. The ECU expects a clean voltage signal that matches the relay's activation state. If the sensed voltage drifts outside expected range or shows instability, the code triggers.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Relay Sense Voltage | Within 0.5V of battery voltage when relay energized; near 0V when de-energized | Voltage outside expected range or fluctuating erratically during relay operation |
| Signal Stability | Clean, stable voltage transitions during relay switching | Noisy signal, slow rise/fall times, or intermittent dropout |
Code P0889 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0889 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.