Gate Select Position Circuit [senses left / right position]
Your transmission's gate select sensor can't tell if the shift mechanism is positioned left or right, like a light switch that won't report whether it's up or down. The ECU can't properly command gear selection because it doesn't know the current position.
The ECM monitors voltage signals from the gate select position sensor to determine left/right shift fork placement. It expects voltage levels to correlate with actual mechanical position; deviation indicates a circuit fault or sensor malfunction. The ECU uses these signals to validate shift commands before executing gear changes.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage | 0.5–4.5V (position-dependent) | Out of range or no signal |
| Position Match | Voltage correlates to actual gate position | Voltage does not match commanded position |
Code P0904 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, P0904 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.