What it means
The transmission control module detected an incorrect ratio while first gear was commanded or expected. The code alone does not confirm slipping or identify the cause.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
The TCM checks first gear by comparing transmission speed signals with the ratio it expects while first gear is commanded. P0731 sets when that first-gear ratio does not match; the code alone does not prove that the first-gear clutch is slipping.
Common causes
Most likely first.
- 1st gear clutch pack worn
- Line pressure low (pump or valve body)
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
- Record freeze-frame data and related transmission codes to confirm the conditions and commanded gear when the first-gear ratio fault set.
- Check fluid level and condition using the vehicle's specified procedure, since low fluid can reduce pressure needed for first gear.
- Inspect speed-sensor wiring and connectors for damage or poor contact that could distort the ratio calculation.
- Compare commanded gear with live input and output speed readings during a controlled test, focusing on the initial launch and first-gear operation.
- Test the speed-sensor circuits and measure transmission line pressure using the manufacturer's procedure before concluding that the first-gear clutch pack is worn.
Common fixes
- Correct low fluid or service fluid that does not meet the vehicle's requirements.
- Repair damaged speed-sensor wiring or loose, corroded connector contacts.
- Replace a faulty speed sensor if testing confirms inaccurate readings.
- Repair a confirmed pump or valve-body pressure fault.
- Repair the first-gear clutch pack if internal wear is confirmed.
Can I keep driving?
If the vehicle slips or cannot pull away reliably in first gear, avoid steep hills, heavy loads, and busy traffic, and arrange prompt diagnosis. Continued slipping can rapidly overheat the fluid and damage the first-gear friction elements.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.