P0729–P076F Gear 6 Incorrect Ratio

Which code is which

Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.

The definition

P0729: Gear 6 Incorrect Ratio.

P0729 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.

What to check first

Cheapest and fastest checks first:

1. Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
2. Check the connector at the part for corrosion, and that it is fully seated.
3. Clear the code and see whether it returns, and under what conditions.

What actually causes this on a VAG

Your gearbox controller constantly compares how fast the engine side of the gearbox is turning against how fast the output to the wheels is turning. Divide one by the other and you get the ratio that is actually engaged. This code means that when sixth was commanded, the number that came back was not the number sixth is supposed to give - so either something slipped, or one of the speed signals used in that calculation is wrong. It can be stored by both twin-clutch DSG gearboxes and by conventional torque-converter automatics, and the parts behind it are quite different in each, so establish which type of gearbox your car has before anyone starts quoting for work.

Most likely causes, in order

1. Gearbox oil overdue, low or degraded. Every clutch and shift element in these boxes is applied by oil pressure, and those pressures are calibrated around fluid in good condition. Tired or low fluid lets an element slip a fraction under load, which is all this plausibility check needs to see. Check the service history for a gearbox oil and filter change first - plenty of owners have never had one done because the box was sold to them as filled for life. A useful clue is whether the fault appears under sustained motorway load in top gear rather than pottering about town.
2. A speed signal problem rather than a mechanical fault. The ratio is a calculation, so a momentary dropout on the input or output shaft speed signal produces a wrong answer from a perfectly healthy gearbox. On DSG boxes these sensors are part of the mechatronic unit rather than separate bolt-on parts. The way to tell the two apart is to log both shaft speeds while driving: a genuine slip shows the ratio drifting away smoothly under load, whereas a signal fault shows the value jumping or dropping to zero for an instant with no change in how the car feels.
3. A problem confined to the sixth gear path. Note carefully whether the car flares, judders or hesitates only in sixth, or in more than one gear - that single observation changes the diagnosis and the bill enormously. On a twin-clutch box the even gears share one clutch, so if sixth misbehaves while the other even gears are clean, a worn clutch pack is a less likely explanation than something in that gear's own engagement path, such as its shift fork or selector not going fully home. If several gears sharing that clutch are affected instead, suspicion moves to the clutch and its hydraulic control.

Worth knowing: Carrying on driving while something slips is what turns a cheap job into an expensive one: friction material shed by a slipping element contaminates the oil and then the mechatronic, so what could have been a fluid service becomes a clutch replacement. A gearbox that slips can also change what the car does under power without much warning, so if it flares or hesitates in sixth, hold a lower gear in manual mode and get it looked at rather than living with it. Have the gearbox adaptation and clutch wear values read before anyone quotes you for a replacement box.

Common questions

What does P0729 mean?

P0729 means Gear 6 Incorrect Ratio. It is logged when the ECU sees this circuit outside its expected operating window.

Can I drive with P0729?

The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.

Related reading

• DSG juddering or jerking: what's normal and what isn't
• DQ200 7-Speed Dry DSG: Problems and What to Check
• DQ250 6-Speed Wet DSG: Problems, Servicing and Buying Checks
• DQ500 & DL501 S tronic: Problems, Servicing, What to Check
• Why the same fault code has completely different causes