P0731: Gear 1 Incorrect Ratio
P0731 means Gear 1 Incorrect Ratio. The transmission control module compares input and output shaft speeds. This code means the measured ratio in first gear does not match the expected ratio, which usually indicates slipping in first gear.
Can I keep driving? Limit driving and diagnose promptly. Continued driving can cause damage, stalling or limp mode.
Common symptoms
- Check engine light on
- Slipping (RPM flares without acceleration)
- Delayed engagement or harsh shifts
- Limp mode
Possible causes
- Low or degraded transmission fluid
- Worn clutches or bands
- Faulty shift solenoid or valve body
- Low line pressure
- Faulty speed sensor
How to diagnose P0731
- Check fluid level and condition (burnt smell, debris).
- Check for solenoid or speed sensor codes.
- Check line pressure.
- Internal repair may be needed if fluid and electrical checks are fine.
What the code means, digit by digit
Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.
Related codes
General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.