P0904: Gate Select Position Circuit A

What it means

The gate select position circuit A reports a circuit a fault. Selector gate position cannot be reliably measured.

At a glance

• Usually lights the check-engine light
• The engine or gearbox may go into limp mode

How it works

Gate Select Position Circuit A reports which shift gate the selector is in on an automated manual transmission. The transmission controller monitors this circuit to identify the selected gate, and this code sets when Circuit A has an electrical fault that makes the position reading unreliable.

Common causes

Most likely first.

1. Position sensor failed
2. Wiring fault

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

1. Record freeze-frame data and view gate select position data on a scan tool while moving the selector through its gates, noting whether Circuit A is fixed, missing, or erratic.
2. Inspect the Circuit A sensor connector and harness for loose or corroded terminals, damaged wiring, or a poor connection near the selector.
3. Check sensor supply, ground, and Circuit A signal continuity against the wiring diagram and manufacturer's specification.
4. If wiring is intact, test the gate select position sensor through its operating range using the manufacturer's procedure.
5. If the sensor signal is correct but the reported gate remains wrong, inspect the selector linkage and gate mechanism for looseness or misalignment.

Common fixes

• Clean and secure the gate select sensor connector.
• Repair damaged or loose Circuit A wiring.
• Replace a failed gate select position sensor.
• Correct loose or misaligned selector linkage if it prevents accurate gate detection.

Can I keep driving?

Avoid driving if the transmission cannot identify the selected gate, shifts are delayed or missing, or limp mode engages; the vehicle may select the wrong gear or lose drive. If symptoms are present, stop safely and arrange prompt diagnosis.