What it means
The transmission control module detected an incorrect ratio while sixth gear was commanded or expected. This can result from transmission slip or an inaccurate speed signal; the code alone does not confirm internal slipping.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
The TCM compares input and output speed to determine whether the transmission is delivering the expected ratio in sixth gear. P0729 sets when the measured ratio does not match the expected sixth-gear ratio while sixth is commanded or expected; it does not by itself prove internal clutch slip.
Common causes
Most likely first.
- Shift solenoid stuck
- Clutch pack worn or slipping
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 check which gear was commanded, along with live input and output speeds, when the ratio fault occurred.
- Check transmission fluid level and condition using the vehicle manufacturer's procedure, and look for leaks or signs of overheating.
- Review related gear-ratio, speed-sensor, and shift-solenoid codes to see whether the sixth-gear error may reflect a bad speed signal or control fault.
- Inspect accessible transmission wiring and connectors for damage, corrosion, or poor contact at the shift-solenoid and speed-sensor circuits.
- If the speed signals and wiring are sound, test sixth-gear solenoid operation and hydraulic or internal transmission function to the manufacturer's specifications.
Common fixes
- Correct a low fluid level and repair any leak using the manufacturer's fluid and service procedure.
- Repair damaged wiring or connector faults affecting the sixth-gear solenoid or speed signals.
- Replace a shift solenoid only if circuit and operation tests confirm it is stuck or faulty.
- Repair worn or slipping internal clutch components if transmission testing confirms internal slip.
Can I keep driving?
Avoid hard acceleration, towing, and continued driving if the transmission slips or shudders, since repeated slip in sixth gear can create heat and worsen damage. If it is in limp mode or cannot shift safely, stop driving and arrange assistance.
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.