P0729
PowertrainGear 6 Incorrect Ratio
Your automatic gearbox tried to put the car into sixth gear, but the speed it measured coming out the other end didn't match the ratio it expected for that gear. The control module reads input and output shaft speeds, does the maths, and if sixth gear is slipping or simply not engaging properly, it sets P0729. For you that usually means a gearbox that won't sit happily at motorway speed, and it's a fault you want diagnosed sooner rather than later before the slipping does real damage.
ⓘ Information only. This page provides general educational information about fault code P0729. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0729 mean?
P0729 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Gear 6 Incorrect Ratio.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Gearbox or engine warning light on, sometimes with a flashing gear display on the dash
- • Won't hold sixth gear on the motorway, either refusing to shift up or dropping back down
- • Revs climb but the car doesn't pull, the classic feel of a gear slipping under load
- • Harsh or delayed upshifts, the box thumping into gear instead of going smoothly
- • Car drops into limp mode and locks you out of the higher gears
- • Fuel economy falls away noticeably, particularly on longer motorway runs
Possible causes
Causes commonly associated with P0729, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Low or burnt automatic transmission fluid, the first thing to rule out and the cheapest to fix. Old fluid loses its grip and the clutches start slipping
- 2. Sticking or failed shift solenoid for sixth gear, so the box can't apply the right clutch pack properly
- 3. Worn internal clutch packs or bands, which is what causes genuine slip once fluid is ruled out
- 4. Clogged transmission filter or strainer choking the hydraulic supply to the gearbox
- 5. Faulty input or output speed sensor feeding the module a duff ratio reading when the gear is actually fine
- 6. Valve body or torque converter trouble, the more involved internal failures that point at a rebuild
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Check the fluid level and condition before anything else. Pull the dipstick or check the level plug, and if the fluid is dark brown, smells burnt, or is plain low, you've found a strong lead
- 2. Scan for all stored codes, not just this one. Speed sensor faults and other ratio codes (P0731 to P0734) sitting alongside it change the whole picture
- 3. Road test it and try to force sixth gear under steady load. Confirm whether it slips, won't engage, or shifts but throws the code regardless
- 4. Watch live data on a scan tool, comparing commanded gear against actual gear ratio and the two shaft speed sensors. That tells you slip versus a lying sensor
- 5. Inspect the wiring and connectors to the shift solenoids for corrosion, road salt damage, or a chafed loom, common on cars that live in the British weather
- 6. If the electronics and fluid check out, a hydraulic pressure test is the next step, and that's specialist gearbox territory
Common questions about P0729
If I clear the code, will it stay gone or just come straight back? +
It'll come back, usually within a drive or two, as soon as the box tries sixth gear and the ratio fails again. P0729 is a live mechanical or hydraulic fault, not a one-off glitch, so clearing it only buys you a few miles of warning light off before it relogs. The only time clearing it sticks is if you've actually fixed the cause first, say a fresh fluid and filter service that sorted a marginal slip.
What am I risking if I just keep driving on it? +
Slipping clutches generate heat, and heat cooks the fluid and wears the friction material faster. What starts as a fluid-and-filter job can turn into a full rebuild if you ignore it for months. There's also the safety side, since the box can drop into limp mode without much warning and leave you stuck in a low gear on a slip road or motorway.
How quickly do I actually need to deal with this? +
Treat it as soon as you reasonably can rather than something you sit on for weeks. If the car is still driving and the fluid turns out to be low or old, getting that sorted quickly might save the gearbox entirely. If it's already slipping hard or dropping into limp mode, stop driving it for long trips and get it looked at before you do expensive damage.
Is this the gearbox itself, or could it just be a sensor or wiring fault? +
Both are possible, which is why diagnosis matters. A faulty output speed sensor or a corroded solenoid connector can report a bad ratio when the gear is mechanically fine, and that's a relatively cheap fix. Genuine slip from worn clutches or a failing valve body is the expensive end. Live data showing whether the gear is actually slipping, versus the sensor lying about it, is what separates the two.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →