Low, degraded or contaminated transmission fluid; Faulty input or output speed sensor, or a wiring or connector fault in their circuits; Worn clutch pack or band for sixth gear, causing internal slip; Shift solenoid or valve body fault, with sticking or low pressure in the sixth gear circuit; Torque converter clutch slip or lock-up fault, depending on the transmission design; Outdated or faulty transmission control software, where the manufacturer has issued a known update; Mechanical damage inside the gearbox, such as a worn gear set or damaged shaft.

P0729

Gear 6 Incorrect Ratio.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
7: AreaTransmission
29: FaultThe specific fault in that area

What it means

P0729 means the transmission control module has detected that the actual ratio between engine speed and output shaft speed does not match the expected ratio when sixth gear is selected. This is calculated from the input and output speed sensor signals. The module sees slip or an incorrect ratio, so it sets the code and may limit gear selection.

Symptoms

Gearbox slipping or engine revving freely in sixth gear
Gearbox dropping out of sixth or refusing to select it
Warning light on, possibly with a transmission warning or limp mode
Harsh or delayed shifts into or out of sixth
Poor fuel economy at motorway speeds
Speedometer or rev reading appearing inconsistent in top gear

Possible causes

Low, degraded or contaminated transmission fluidlikely
Faulty input or output speed sensor, or a wiring or connector fault in their circuitslikely
Worn clutch pack or band for sixth gear, causing internal slippossible
Shift solenoid or valve body fault, with sticking or low pressure in the sixth gear circuitpossible
Torque converter clutch slip or lock-up fault, depending on the transmission designpossible
Outdated or faulty transmission control software, where the manufacturer has issued a known updaterare
Mechanical damage inside the gearbox, such as a worn gear set or damaged shaftrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including speed, load, gear and fluid temperature
Check for related codes covering other gear ratios, speed sensors, solenoids or pressure faults, as these point towards the root cause
Check the transmission fluid level and condition where a dipstick or check procedure is available, following the manufacturer's method for the model
Inspect the speed sensor wiring and connectors for damage, corrosion, chafing or fluid ingress
Compare live input and output speed sensor data with engine speed and road speed during a road test, looking for slip in sixth only
Check the transmission's live data for solenoid commands, line pressure and torque converter slip where available
Check for any applicable software updates or technical bulletins for the vehicle
Carry out pressure tests or a gearbox inspection if the electrical and fluid checks find nothing, as the procedure varies by transmission

How to fix

Correct the fluid level or change the fluid and filter using the specified type, if it is low or degraded
Repair damaged wiring or connectors, or replace a faulty speed sensor
Replace or repair a faulty shift solenoid or valve body
Overhaul or replace the transmission if the sixth gear clutch or internal parts are worn or damaged
Update the transmission control software only if a known fault with the existing calibration is confirmed

Can I keep driving?

Medium severity

The car can usually be driven for a short time, ideally avoiding sixth gear, but continued slip can quickly cause expensive internal damage. Get it diagnosed soon, and stop driving if the gearbox slips badly or goes into limp mode.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.