Fix P0734: Mechanic’s solutions that WORKED

The code P0734: Gear 4 Incorrect Ratio points out a fault in the fourth gear of the vehicle’s transmission, indicating that the ratio of the gear’s operation is incorrect. This can be due to various reasons, such as internal wear, fluid issues, or electronic malfunctions. It is a critical code as it directly affects the vehicle’s transmission, which is essential for its movement and operation.

P0734 quick overview

Meaning	P0734: Gear 4 Incorrect Ratio
Is it serious?	Yes, this code is serious as it indicates a problem with the transmission, which can lead to drivability issues and potential transmission damage.
Possible causes	– Worn or damaged gear components – Low or dirty transmission fluid – Faulty shift solenoids – Transmission control module issues – Internal mechanical problems in the transmission
How to diagnose?	Check Transmission Fluid: Verify the level and condition of the transmission fluid. Inspect Gear Components: Look for wear or damage to the gears. Evaluate Shift Solenoids: Test the operation of transmission shift solenoids. Scan for Additional Codes: Use a diagnostic scanner to check for other related transmission codes. Pressure Testing: Perform transmission pressure tests to identify internal leaks. Inspect TCM: Check the Transmission Control Module for faults or programming issues.

How to fix P0734 – learn from mechanics

I looked into P0734 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Volvo

Vehicle Information: 2000 Volvo S40 1.9L

Issue + Repair History:

• Engine light on with codes P1618 and P0734 (gear 4 incorrect ratio).
• Cleared codes and road-tested, car performed well.
• Customer experienced the same issue the next day.

Suggestions to Fix Issue:

• Consider internal component wear as a common cause for gear ratio code.
• Recommend a full transmission flush as an initial step.
• Check for issues with transmission speed sensor signal.
• Examine Solenoid S1 and S2 circuits for contact resistance.
• Verify gear-shift position sensor adjustment.
• Ensure transmission oil level is correct.
• Look for mechanical faults in the transmission.
• Inspect transmission speed sensor and solenoid circuit resistance.

Final Fix:

• Changed fluid as an initial step.
• Noted the condition of the fluid indicated the problem might be internal.
• Fixed by replacing gear position sensor

Case #2: Jeep

Vehicle Information: 2002 Jeep Grand Cherokee Overland 4.7L

Issue + Repair History:

• Engine light on with codes PO734 and P1790, and burnt fluid.
• After rebuilding transmission, the problem persisted with the vehicle moving forward in park and neutral.

Other OBD2 Codes + Test Results:

• PO734
• P1790

Suggestions to Fix Issue:

• Disconnect transmission solenoid connector to check for internal mechanical problems.
• Re-examine the transmission, focusing on the oil pump, pressure control valve, and solenoid.
• Inspect the separator plate in the oil pump for signs of damage (“gun barrel blue”).
• Conduct line pressure testing to isolate the problem area.
• Check for issues with the valve body affecting the manual valve and park rod.
• Consider the possibility of a locked torque converter or a faulty switch on the valve body.

Final Fix:

• Discovered that the friction plates in the underdrive section were not properly set, with insufficient clearance. Adjusting this resolved the issue.

Case #3: Mitsubishi

Vehicle Information: 2004 Mitsubishi Galant GTS 3.8L

Issue + Repair History:

• Delayed gear engagement from park to reverse and drive.
• Felt as though it went into neutral when coming to a stop.
• Code P0734 stored in computer.
• Transmission was rebuilt, but the problem persisted.
• Replaced input speed sensor, but it didn’t solve the issue.

Other OBD2 Codes + Test Results:

• P0734

Suggestions to Fix Issue:

• Check if the P0734 code was cleared or if it reappeared after repair.
• Try resetting the transmission adaptives by disconnecting and clamping the battery cables together.
• Consider the possibility of low transmission fluid; add a quart to see if it changes the condition.
• Ensure the correct type of transmission fluid is used, given the recent rebuild.

Final Fix:

• Returned the car to the transmission rebuilder for rechecking.
• The issue was with the electronic pressure control solenoid failing.
• All solenoids were replaced, resolving the shift problems.

Case #4: Nissan

Vehicle Information: 2008 Nissan Xterra SE 4.0L

Issue + Repair History:

• SUV came from a body shop after minor front damage; cooling fan, radiator, and trans cooler were replaced.
• Transmission ran low on fluid and burned the forward clutches.
• Transmission was rebuilt, but valve body and TCM were not replaced initially.
• Vehicle returned with codes P0717 (input turbine sensor) and P0734 (gear 4 incorrect ratio).
• Valve body and TCM replaced and programmed, but the codes persisted.
• Transmission shifted through all gears perfectly.

Other OBD2 Codes + Test Results:

• P0717: Input turbine sensor.
• P0734: Gear 4 incorrect ratio.

Suggestions to Fix Issue:

• Check for non-erasable codes.
• Consider mechanical faults in the transmission if all wiring is okay.
• Investigate TCM issues if 4th gear does not slip while driving.
• Look into potential sticky valve in the valve body.
• Consider installing a shift kit from Sonnax.
• Explore replacing the input speed sensor.

Final Fix:

• Discovered the torque converter had different stall speeds.
• Identified that one of the molded pistons was nicked, causing the input clutch to drag.
• Replacing the torque converter and addressing the piston issue resolved the codes.

Case #5: Mazda

Vehicle Information: 2000 Mazda 626 ES 2.0L

Issue + Repair History:

• Initially came in with transmission slipping.
• Rebuilt transmission; worked well for over a month.
• Returned with an issue of dropping out of overdrive once warmed up.

Other OBD2 Codes + Test Results:

• P1744
• P0733
• P0734

Suggestions to Fix Issue:

• Initially suspected a problem with the torque converter.
• Considered the possibility of an internal leak causing pressure loss.
• Suggested checking and replacing the transmission range switch, valve body, and torque converter.
• Recommended monitoring the input speed sensor.
• Advised installing a transgo kit in the transmission.
• Examined the servo and band for proper functioning.

Final Fix:

• Replaced the torque converter, which did not resolve the issue.
• Sent the vehicle to a reputable local transmission shop.
• Diagnosed with an internal leak causing pressure loss, leading to dropping into second at highway speeds.
• Replaced the transmission, resolving the problem.

Advice	Brief Description of Diagnostic Step
Check and Replace Torque Converter	Inspect the torque converter for internal leaks or damage.
Inspect Valve Body	Examine the valve body for issues that could be causing problems.
Evaluate Solenoids and Electrical Parts	Check transmission solenoids and related electrical components.
Monitor Transmission Fluid Condition	Assess the condition and level of the transmission fluid.
Perform Pressure Testing	Conduct pressure tests to identify internal transmission leaks.
Reset and Recalibrate Transmission	Reset transmission settings and recalibrate if necessary.
Inspect for Mechanical Faults	Look for any mechanical faults within the transmission.
Consider Transmission Rebuild or Replacement	Evaluate the need for a transmission rebuild or replacement.