What P0729 means
The transmission module divides input (turbine) speed by output speed and compares the result with the ratio of the commanded gear; a deviation means slip – a clutch or brake not holding, too little pressure – or a wrong gear.
How the engine computer detects it
Thresholds: Ford more than 30 rpm off for 1.8 s three times (diesel 2023: over 20 % for 12 s), GM's 2006 tables and the Allison 1000 converter slip of 200–230 rpm for 2.25 s after one trip, Saab's Aisin boxes over 20 % twelve times without MIL. P0729 concerns sixth gear. Saab's Aisin six-speed also checks whether the actual ratio matches fourth or fifth – the sixth gear clutch is not holding and the box drops into another gear. Ford stores it only if the fault does not fit a particular solenoid (otherwise P0751 ff). On GM manual transmissions (LT4) it checks the gear position sensor instead.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Slipping or flare in sixth gear, engine speed higher than expected
- Harsh or delayed shifts
- Limp mode (Aisin: second gear and reverse only), transmission warning
- Check engine light depending on manufacturer
Common causes
- Fluid level too low or fluid worn or contaminated Most likely
- Clutch for sixth gear worn (linings, piston seal) Most likely
- Sticking shift or pressure valve, leaking valve body gasket, blocked passages Possible
- GM manual transmissions only: gear position sensor faulty or not learned Possible
How to diagnose P0729
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: transmission clutches, transmission valve body, transmission fluid, transmission control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 7
- Transmission
- 29
- Specific fault
Questions drivers ask
Can I drive with code P0729?
If the car shifts normally, drive gently to a shop. If it's stuck in one gear, slips, shifts harshly or won't move out of Park, don't drive it: have it towed.
Will my car pass an emissions inspection with P0729?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.