Fix P0731: Mechanic’s solutions that WORKED

The code P0731: Gear 1 Incorrect Ratio indicates a problem with the first gear of your vehicle’s transmission, specifically that the gear ratio is incorrect. This could mean there’s a mechanical issue inside the transmission or a problem with the sensors or control module that manages gear shifting. It’s a significant issue because it affects how the car drives, particularly during gear shifts, and needs prompt attention to prevent further damage.

P0731 quick overview

Meaning	P0731: Gear 1 Incorrect Ratio
Is it serious?	This code is quite serious as it indicates a malfunction in the transmission’s first gear, which can lead to drivability issues.
Possible causes	– Faulty or slipping transmission – Issues with the transmission control module – Sensor malfunctions (input or output speed sensors) – Mechanical wear inside the transmission
How to diagnose?	Check Transmission Fluid: Ensure it’s at the correct level and condition. Inspect Speed Sensors: Verify the operation of input and output speed sensors. Evaluate Transmission Control Module: Check for any issues with the module. Scan for Additional Codes: Look for other codes that might indicate related problems.

How to fix P0731 – learn from mechanics

I looked into P0731 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: Ford

Vehicle Information:
1996 Ford Taurus LX 3.0L

Issue + Repair History:

• Customer complained of extreme loss of power, unwilling to drive the car.
• The only trouble code was P0731 related to transmission gear selection.
• The Overdrive (O/D) lamp flashed at least once.

Other OBD2 Codes + Test Results:

• P0731: 1st gear failure – incorrect gear ratio while commanding 1st gear

Suggestions to Fix Issue:

• Considering replacing the Transmission Range Position Sensor.
• Verify the functionality of the transmission and its components.

Final Fix:

• Replacing the Transmission Range Position Sensor resolved the issue after extensive test driving.

Case #2: Chrysler

Vehicle Information:
2002 Chrysler 300M 3.5L

Issue + Repair History:

• The vehicle received a used transmission installation and exhibited code P0731.
• After input sensor replacement, the car is in limp mode with the same code.

Other OBD2 Codes + Test Results:

• P0731 – Input sensor issue.

Suggestions to Fix Issue:

• Check the compatibility of the transmission with the vehicle model.
• Inspect wiring between the sensor and the transmission control module.
• Verify if the correct sensors are installed and functioning.

Final Fix:

• Client replaced the transmission and wire harness with another used set.
• Discovered multiple bare wires in the old wire harness. Fixing these resolved the issue.

Case #3: Nissan

Vehicle Information:
2001 Nissan Sentra GXE 1.8L

Issue + Repair History:

• Vehicle presented with stalling issues, running rough, and MIL on.
• Found code PO731, which was cleared and hasn’t reappeared.
• Performed tune-up: replaced spark plugs, air filter, and serviced transmission.
• Vehicle continues to stall after warming up.

Other OBD2 Codes + Test Results:

• PO731 – 1st gear malfunction

Suggestions to Fix Issue:

• Consider checking the Idle Air Control (IAC) valve, common issue in Nissans.
• Inspect and possibly replace the Mass Air Flow (MAF) sensor.
• Examine ECM and wiring for issues, particularly the power supply to ECM.

Final Fix:

• Replaced the Mass Air Flow sensor and used premium fuel, which improved vehicle performance.

Case #4: Ford

Vehicle Information:
2005 Ford Escape XLT 2.3L

Issue + Repair History:

• After engine replacement, transmission went into limp mode with O/D light flashing.
• Experienced poor performance and erratic shifting.
• Codes P0721, P0731, and P0732 were found.

Other OBD2 Codes + Test Results:

• P0721: Output Speed Sensor Range/Performance
• P0731: Gear 1 Incorrect Ratio
• P0732: Gear 2 Incorrect Ratio

Suggestions to Fix Issue:

• Check if the speed sensors are correctly connected.
• Inspect the wiring and connectors related to the transmission.
• Consider potential internal transmission damage due to the previous engine failure.

Final Fix:

• The issue was resolved by correctly connecting two crossed connectors on the side of the transaxle.

Case #5: Ram

Vehicle Information:
2015 Ram 2500 SLT 6.7L

Issue + Repair History:

• Came in with a replacement transmission from a salvage yard, but symptoms persisted.
• The truck had a transmission neutralization issue and a P0731 code for gear ratio error.
• Initially, the issue occurred during 1st to 2nd gear shift. The vehicle was undriveable due to the frequent occurrence of the problem.

Other OBD2 Codes + Test Results:

• P0731: #1 Gear Ratio Error

Suggestions to Fix Issue:

• Verify if PCM is controlling the pressure control solenoid correctly.
• Consider doing a software update and relearn process for the transmission.
• Check RPM and ISS Turbine shaft speeds.
• Reset transmission adaptives.

Final Fix:

• The resolution involved addressing two separate issues: the PCM and the transmission.
• Replacing the PCM was part of the solution.
• The installed transmission was faulty. Rebuilding the original transmission resolved the remaining problems.

Advice	Diagnostic Step Description
Check Transmission Solenoids	Test the operation and electrical connections of transmission solenoids.
Inspect Transmission Internals	Examine the transmission for any internal damage or wear.
Verify Wiring and Connectors	Check the transmission wiring and connectors for damage or loose connections.
Consider PCM Issues	Evaluate the Powertrain Control Module (PCM) for potential problems.
Perform Software Update and Relearn	Update transmission software and perform a relearn process if necessary.