Technical Description of OBD-II Trouble Code
Incorrect Ratio in Gear 6
This powertrain code is a generic OBD-II trouble code that applies to all makes and models of vehicles (1996-newer). However, the specific repair steps may vary depending on the model.
What does this mean?
Modern vehicles with automatic transmissions use a torque converter to increase engine torque output and drive the rear wheels. This code can appear in vehicles with automatic transmissions when there is a problem shifting into 6th gear or while driving in 6th gear. The computer-controlled automatic transmission uses multiple gear ratios to increase vehicle speed and maximize engine power output. Newer vehicles may have more than four gear ratios to improve fuel economy. The computer determines when to shift up or down between gears based on throttle position and vehicle speed.
The Engine Control Module (ECM), Powertrain Control Module (PCM), or Transmission Control Module (TCM) uses input from various sensors to ensure the correct operation of the transmission and its components. Engine speed is often compared to the transmission speed sensor to determine gear ratio and torque converter slip. If the calculation does not match the desired value, a Diagnostic Trouble Code is set and the Check Engine Lamp is illuminated. Resolving incorrect ratio codes typically requires advanced mechanical skills and diagnostic tools.
Note: This code is similar to P0730, P0731, P0732, P0733, P0734, P0735, and P0736. If there are other transmission codes, it is recommended to repair those problems first before addressing the incorrect gear ratio code.
Symptoms
Symptoms of a P0729 trouble code may include:
Potential Causes
Possible causes of this DTC may include:
Diagnostic & Repair Steps
Always check the fluid level and condition before proceeding with further diagnosis. Improper fluid level or dirty fluid can cause shifting issues that affect multiple gears. If only one incorrect ratio code is displayed, test drive the vehicle to determine if the gear is actually functioning properly.
The transmission’s internal clutches and bands are usually controlled by a solenoid or fluid pressure. If there is an electrical issue with a shift solenoid, a code specific to that fault should also be displayed. Address the electrical issue before proceeding further. Blocked fluid passages inside the transmission can also prevent the vehicle from shifting into 6th gear. If there are multiple incorrect ratio codes but the transmission performs as expected, there could be a mechanical issue with the torque converter, transmission main control, or pressure problems.
If there are multiple incorrect ratio codes and shift issues, but not all are displayed, the fault may be a mechanical issue internal to the transmission or a fluid pressure/blockage problem in the main control. A shift solenoid that controls multiple gears can also be at fault if there is an internal blockage or fluid pressure issue. If there is an electrical problem with the solenoid that controls the gears, a code specific to the faulty solenoid should be displayed.
It may be necessary to use an advanced diagnostics scan tool to determine which gear the transmission is being commanded to engage and to verify if the engine speed matches the calculated output speed from the transmission sensor. Repairing this type of malfunction often requires advanced knowledge of transmission operation and overhaul. Consult the factory service manual for vehicle-specific diagnostic procedures.
