P0731 Gear 1 Incorrect Ratio

P0731 indicates gear 1 incorrect ratio. The transmission is not producing the expected input/output speed relationship for first gear.

Code details

Generic OBD-II · P0

System: Transmission
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not assume first gear is mechanically failed until sensor data, pressure control, and fluid condition are checked.

Symptoms of P0731

Harsh, delayed, or slipping shifts, or being stuck in one gear.
Check-engine light, sometimes with limp-home mode.
Flaring RPM or a shudder under load.

Common causes of P0731

Low or degraded transmission fluid.
A failing speed sensor, shift solenoid, or pressure-control circuit.
Wiring, connector, or internal hydraulic/mechanical faults.

Useful checks before replacing parts

Compare commanded first gear, actual ratio, input speed, output speed, pressure data, and slip during launch.
Check fluid level and condition, solenoid command, pressure-control response, speed sensor data, and clutch apply behavior.
Review whether the code appears only on cold launch, hot operation, or during limp mode.

Related OBD-II codes

These nearby transmission entries can help compare wording and pin down the fault area.