P0734 OBD-II Trouble Code: Gear 4 Incorrect Ratio

P0734 code definition

A P0734 trouble code indicates that an abnormality has been detected in the gear ration of one of the automatic transmissions gears, causing the storage of a code in the Powertrain Control Module (PCM) and illumination of the Check Engine Light. Related codes include P0729, P0730, P0731, P0732, P0733, P0735, and P0736.

What the P0734 code means

Utilizing the position of the throttle, engine load and speed, as well as vehicle speed, the vehicle computer determines the appropriate gear to shift into to maintain the desired speed and power in an automatic transmission. The P0734 trouble code develops when there is a problem shifting into fourth gear specifically.

The vehicle's Engine Control Module (ECM), Transmission Control Module (TCM), or PCM, depending on the vehicle make and model, uses information from the transmission speed sensor, calculated against engine speed, to determine the proper gear ratio. And when this data does not match up, a P0734 trouble code is stored.

What causes the P0734 code?

The causes of a P0734 trouble code include a faulty PCM, ECM, or TCM; blockage inside the transmission; a faulty shift solenoid, dirty or contaminated transmission fluid, or a low transmission fluid condition, and a mechanical transmission failure.

What are the symptoms of the P0734 code?

Symptoms of a P0734 trouble code range from the failure of the transmission to shift into a particular gear to no apparent malfunction. The vehicle could also suffer from the transmission slipping and overheating, a decrease in fuel efficiency, and other gear ratio codes being stored on the PCM in addition to the original code.

How does a mechanic diagnose the P0734 code?

Fixing and clearing a P0734 trouble code requires the use of an advanced OBD-II scanning tool, a manual pressure gauge, and a digital volt/ohmmeter by the mechanic. In addition, the mechanic should perform the following steps when attempting a diagnosis:

• The mechanic should begin the diagnosis by checking the transmission fluid level and condition. While inspecting the fluid, the mechanic should be on the lookout for a burnt or abnormal smell, which can indicate that the transmission has suffered damage from a low fluid condition.

• If this is the case, the mechanic should remove the transmission pan and inspect it for pieces of the clutch or other debris.

• If the mechanic finds no debris, then they should check the wiring and connectors for damage.

• If the wiring and connectors appear to be in good shape, then they should connect the OBD-II scanner to the diagnostic connector and download any stored trouble codes and freeze frame data.

• Additionally, if the transmission fluid is low, the mechanic should fill the transmission fluid to a point that any leaks can be detected and repaired.

• Once all leaks are found and fixed, the transmission fluid must be filled to an appropriate level.

• The mechanic should clear the code, allow the vehicle to reach a normal operating temperature, and test drive the vehicle to see if the trouble code returns.

• If the code returns and the transmission is still slipping or hesitating, the mechanic should connect a manual pressure gauge. Using the manual pressure gauge in conjunction with a hydraulic pressure diagram, they should check the pressure of the transmission's hydraulic pump. The mechanic should compare their findings to manufacturer specifications.

• If the pump checks out and the code still persists, the mechanic should use the digital volt/ohmmeter to test for reference voltage and ground signal at the shift solenoid. They need to then check for circuit continuity, making sure to disconnect any related control modules to prevent memory loss.

• Furthermore, the mechanic should use a manufacturer's wiring diagram to test all related circuits for resistance and continuity, comparing their findings to the manufacturer's specifications.

• If everything checks out and is in good condition, the next step is to diagnose the PCM for failure.

• Finally, once all repairs are made, the mechanic should clear the code and retest the system to see if the trouble code returns.

Common mistakes when diagnosing the P0734 code

A common misdiagnosis by a mechanic is to blame the shift solenoid and replace that component. Ultimately, this does not fix the problem and the condition returns.

How serious is the P0734 code?

The condition that causes the storage of a P0734 trouble code, if left untreated, can lead to the transmission failure, worsening fuel mileage, and trouble shifting while driving.

What repairs can fix the P0734 code?

Fixing and clearing a P0734 trouble code is a multi-step process and requires the mechanic to perform the following:

• If upon inspection it is determined that the transmission has suffered damage due to a low fluid condition, the mechanic must first rebuild the transmission, perform a fluid flush, and replace the torque converter, before the transmission will perform properly.

• If a rebuild and flush does not fix the problem, the problem might lie with the wiring or connectors. In this case, the mechanic should replace any damaged, or faulty, wiring or connectors. Most often, if damaged components are detected during this inspection, the full internal transmission harness needs replacing.

• The mechanic should repair any leaks if the transmission fluid looks like it is in good condition and is only low.

• In addition, the mechanic should replace the hydraulic pump if this component is damaged. Replacing the pump requires the mechanic to remove and partially disassemble the transmission.

• Repair or replace any damaged circuits or connectors.

• Replace the shift solenoid, or solenoid pack if faulty.

• Replace the PCM if found faulty upon inspection.

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