Transmission · Powertrain · generic (SAE-defined)
P0729: Gear 6 Incorrect Ratio
P0729 means the transmission computer measured an input-to-output speed ratio in sixth gear that does not match the ratio sixth gear should produce, indicating slipping or a sensor problem.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0729 sets
When sixth gear is commanded, the transmission control module (TCM) divides input shaft speed by output shaft speed and compares the result with the known sixth-gear ratio. If the measured ratio stays outside an allowed band for a set time, the TCM sets P0729. The allowed band and timing are manufacturer-specific.
Common causes of P0729
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Low or degraded transmission fluid
- Worn clutches or bands applied in sixth gear
- Faulty shift or pressure control solenoid
- Sticking valve in the valve body
- Input or output speed sensor giving incorrect data
- Internal leak from worn seals
P0729 symptoms
- Check engine light on
- Engine speed flares or rises in sixth gear
- Transmission will not stay in or shift into sixth
- Limp mode
- Reduced highway fuel economy
How P0729 is diagnosed
- Read all transmission codes and record freeze frame data
- Check fluid level and condition using the manufacturer's procedure
- Road test while monitoring input, output and engine speed in sixth gear
- Verify both speed sensors read correctly before condemning hard parts
- Check solenoid operation and pressure control commands
- Perform pressure testing or a valve body inspection if needed
Typical fixes for P0729
- Correct fluid level or service the fluid and filter
- Replace faulty shift or pressure control solenoids
- Repair or replace the valve body
- Replace a faulty speed sensor
- Overhaul or replace the transmission if clutches are worn
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Related codes & warning lights
P0729 FAQ
What does code P0729 mean?
P0729 means the transmission computer measured an input-to-output speed ratio in sixth gear that does not match the ratio sixth gear should produce, indicating slipping or a sensor problem. The standard definition is "Gear 6 Incorrect Ratio."
Can I drive with a P0729 code?
Minimize driving. Slipping in sixth gear generates heat and wears clutches quickly, and the transmission may enter limp mode, so avoid highway cruising and heavy loads until repaired.
Does P0729 mean my transmission needs rebuilding?
Not automatically. Low fluid, a solenoid or a sensor fault can cause it, so those simpler items should be checked before internal repairs are considered.
Why does this only happen in sixth gear?
Each gear uses a particular combination of clutches. A worn or poorly applied clutch that is only used in sixth gear will only cause slipping there.
Keep researching
More transmission codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.