Error Code P0729 is defined as Gear 6 Incorrect Ratio. This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model, and powertrain configuration.

Modern vehicles equipped with automatic transmissions/transaxles use a torque converter between the engine and transmission, which increases the engine torque output and drive the rear wheels.

When a vehicle with an automatic transmission experiences a problem shifting into the 6th gear, or experience problem while driving in 6th gear, then Error Code P0729 will be registered. The computer controls the automatic transmission, and it comes with multiple gear ratios that increase the vehicle’s speed, while simultaneously maximizing the power output from the engine. Newer vehicle models, of course, come with more than four gear ratios to improve their fuel efficiency. With this, the computer determines the best time to shift gear (up or down) based on throttle position versus vehicle speed.

The PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) or TCM (transmission control module), uses input from different sensors to verify the correct operation of the transmission components. Engine speed is usually calculated against the transmission speed sensor to determine gear ratio and torque converter slip. If the calculation doesn’t match the desired value, then an error code like this one will show up and activate the Check Engine light. Incorrect ratio codes will usually require advanced tools and knowledge to diagnose accurately.

Other related error codes include:

If there are other transmission codes present, then it’s important to repair these problems before proceeding on addressing incorrect gear ratio code.

Common Symptoms

  • Delayed shifting or not shifting to 6th gear
  • Increase in fuel consumption
  • Transmission slipping

Possible Causes

  • Dirty or low transmission fluid
  • An internal blockage inside transmission main control
  • Mechanical failure inside the transmission
  • Defective transmission control module

How to Check

When diagnosing this code, make sure you check the fluid level and its condition. Dirty or improper fluid level can cause shifting problems that may affect multiple gears. If there is only one incorrect ratio code shown, then take the vehicle for a test drive to determine which gear is working properly.

A solenoid usually controls internal clutches and bands of the transmission or using fluid pressure. If there’s an electrical issue with the shift solenoid, then a code specific to that fluid must be shown. Make sure you correct the electrical problem before proceeding in your diagnosis. Blocked fuel passage within the transmission can keep the vehicle from shifting to the 6th gear. If there are many wrong ratio codes, but the transmission works the way it should, then there could be a mechanical problem within the torque converter, transmission main control, or pressure problems.

If there are multiple wrong ratio codes and shift issues, but not all are displayed, then the problem could be mechanical within the transmission, fluid pressure, or blocked passage in the main control. A shift solenoid that controls multiple gears can also be a problem if there’s an internal blockage or problem in the fluid pressure. If there’s an electrical problem within the solenoid that controls the gears, then a code must be displayed specific to the defective solenoid.

In most cases, advanced diagnostic tools are necessary to pinpoint which gear is being commanded by the transmission, and determine the engine speed to match with the calculated output speed from the transmission sensor. With this, advanced knowledge of transmission operation and overhaul may be necessary. Thus, it’s crucial to consult with your factory service manual for the specific diagnostic procedure of the fault code.

How to Fix

Depending on the diagnosis, common repairs for this code include:

  • Changing of transmission fluid and filter
  • Repair or replacement of damaged connector and wirings
  • Repair or replacement of damaged transmission or torque convertor components

In most cases, this code is triggered by dirty fluid, or not enough transmission fluid. Thus, it’s important to check it first.

Also, sometimes, a simple service for the transmission will fix this problem. Though this isn’t always the case, proper maintenance can prevent this error code from happening.