What does it mean?
Modern vehicles equipped with automatic transmissions/axle transmissions use a torque converter between the engine and the transmission to increase engine torque and drive the rear wheels.
This code may appear in vehicles with an automatic transmission when there is an issue shifting into 4th gear or while driving in 4th gear. The computer-controlled automatic transmission uses multiple gears to increase vehicle speed while maximizing engine power output. Newer vehicles may have more than four gears to improve fuel economy. The computer determines when to upshift or downshift between gears based on throttle position relative to vehicle speed.
The Engine Control Module (ECM), Powertrain Control Module (PCM), or Transmission Control Module (TCM) uses inputs from various sensors to verify proper transmission and component operation. Engine speed is often calculated relative to the transmission speed sensor to determine gear ratio and torque converter slippage. If the calculation does not match the desired value, a diagnostic code is set and the Check Engine light is illuminated. Incorrect ratio codes typically require advanced mechanical capability and diagnostic tools.
Note: This code is similar to P0730, P0731, P0732, P0733, P0735, and P0736. If other transmission codes exist, repair those issues before proceeding with an incorrect gear ratio code.
Symptoms
Symptoms of a P0734 trouble code may include:
Illuminated check engine light (malfunction indicator)
Delayed or no shift into 4th gear
Transmission slipping
Reduced fuel economy
Potential Causes
Causes of this DTC may include:
Low or dirty transmission fluid
Mechanical failure inside the transmission
Internal blockage within the main transmission control
Faulty shift solenoid
Faulty transmission control module
Diagnostic and Repair Steps
Always ensure to check the fluid level and condition before proceeding with diagnosis. Incorrect fluid level or dirty fluid can cause shifting problems affecting multiple gears. If only one incorrect ratio code is displayed, test drive the vehicle to determine if the gear is actually functioning.
Transmission internal clutches and bands are typically controlled by a solenoid or using fluid pressure. If there’s an electrical issue with a shift solenoid, a specific fault code for that should also be displayed. Address the electrical issue before proceeding. Blocked fluid passages inside the transmission can also prevent the vehicle from shifting into 4th gear. If multiple incorrect ratio codes exist but the transmission operates as expected, there might be a mechanical issue with the torque converter, main transmission control, or pressure problems.
If there are multiple incorrect ratio codes and shifting problems, but not all are displayed, the fault could be an internal mechanical problem within the transmission, or fluid pressure/blocked passages in the main control. A shift solenoid controlling multiple gears could also be responsible in case of internal blockage or fluid pressure issues. If there’s an electrical problem with the solenoid controlling the gears, a code specific to the faulty solenoid should be displayed.
It may be necessary to use an advanced diagnostic scan tool to determine which gear is being commanded by the transmission and to determine if engine speed matches the calculated output speed from the transmission sensor.
Comprehensive knowledge of transmission operation and overhaul is often required to repair this type of malfunction. Consult the factory service manual for vehicle-specific diagnostic procedures.