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Code P0731. Incorrect Gear Ratio 1

The code P0731 is described as: Incorrect gear ratio 1. This means there is a problem with the automatic transmission in first gear, usually caused by problems with the solenoid or dirty transmission fluid.

This is a fault code that applies generically to any brand of car, since it is part of the OBD-II system. And although it may have different definitions depending on the brand, they all refer to the same failure.

Automatic Transmission Operation

Does the car’s transmission continually slip along with slow acceleration? Is there a problem of decreased fuel economy along with the vehicle not starting in 1st gear? Is the car check engine light on the dashboard on?

These are all an indication of a problematic transmission system and incorrect gear ratio, resulting in the P0731 error code being stored in the powertrain control module (PCM).

Modern vehicles that come equipped with automatic transmissions or transaxles use a torque converter between the engine and transmission to enhance the engine’s torque output and drive the rear wheels.

The PCM (powertrain control module) controls the automatic transmission by using multiple gear ratios and increases vehicle speed to maximize engine power output.

Newer vehicle models are equipped with more than four gear ratios. The PCM decides when to shift up or down between gears, based on vehicle speed and throttle position.

It does this by using input from various sensors to verify the proper operation of the transmission and all of its components. The P0731 error code occurs when there is a problem shifting into the sixth gear or driving in the first gear.

If there are other problems with the transmission code, they must be fixed first before fixing the P0731 code.

You may also be interested in Error Code P0725: What Does It Mean and What to Do?

What Is Code P0731?

Fault code P0731 is called “Incorrect gear ratio 1“, but in different programs, it may be called differently. This fault designation applies to all OBD-II-equipped vehicles.

This code may appear on vehicles with an automatic transmission when there is a problem shifting into or out of 1st gear.

A computer-controlled automatic transmission uses multiple gear ratios to increase vehicle speed while maximizing engine output.

Newer cars may have more than four gear ratios to improve fuel economy. The computer determines when to shift up or down based on the throttle position and vehicle speed.

Engine speed is often calculated from the transmission speed sensor to determine torque converter ratio and slip.

If the calculation does not match the desired value, a P0731 OBD2 diagnostic trouble code is set and the check engine light illuminates. Incorrect ratio codes usually require a mechanical and diagnostic scanner check.

Common Symptoms of Code P0731

As with other error codes, P0731 triggers the Check Engine light. Common symptoms of this error code may include the following:

  • Delayed shifting or the vehicle does not shift into the correct gear.
  • Slow acceleration.
  • Do not start in 1st gear.
  • Sliding transmission.
  • Decreased fuel efficiency.
  • MIL Light was activated.
  • Transmission overheating.

Possible Causes of Code P0731

The P0731 fault code can be caused by multiple factors, such as:

  • Dirty transmission fluid or insufficient level.
  • Mechanical failure within the transmission.
  • Clogged fluid filter or worn pump
  • Internal lock on main transmission control.
  • Solenoid, torque converter clutch or internal lockup.
  • Faulty shift cables or solenoids.
  • Faulty transmission control module.
  • Defective torque converter clutch.
  • Clogged transmission fluid passages.

How to Diagnose the P0731 Code

Before starting the diagnosis, you must first check the level and quality of the transmission fluid. Old, dirty fluid must be replaced. Do not make the mistake of topping up the level with new fluid, if you find that it is dirty, dark, and in poor condition, you should change it.

To begin diagnosing this error code, an OBD-II scanner is used to view real-time information. Check this and other codes that are recorded, and take note of all of them, it can help you relate the failure.

Then clear the codes and take a test drive. If the code returns, then the problem is really due to an actual failure in the transmission.

The internal transmission bands and clutches are usually controlled by the solenoid or by fluid pressure. Any electrical problem with a shift solenoid can cause the error code.

Therefore, it is important to troubleshoot electrical problems before proceeding to the next diagnostic step. Additionally, a blocked fluid passage within the transmission can prevent the vehicle from starting in first gear.

If there are multiple related codes, but the transmission is performing as expected, the problem could be a mechanical problem within the torque converter, a pressure problem, or a transmission main control problem.

If there are multiple activated codes and shift problems, but not all are displayed, the fault may be an internal mechanical problem with the transmission, fluid pressure, or blocked passage in the main control.

Depending on the severity of the problem, technicians may perform a torque converter stall speed test to determine whether the engine stall speed is within factory specifications or not.

The engine must be able to match the transmission sensor readings.

How to Repair Code P0731

According to the diagnosis result, the repairs for this error code are as follows:

  • Check all electrical connections and cables connecting the sensors to the transmission control module.
  • Repair or replace damaged or corroded cables in and around the transmission control module.
  • If the transmission fluid and wiring are in good condition, the transmission components should be checked for problems and repaired, or replaced if necessary.
  • If there are problems with the shift solenoids, pump, or torque converter, the transmission must be disassembled to properly repair these components.
  • After making any necessary repairs, you should assemble and test the transmission to make sure everything is working properly without any fault codes activated.

The transmission problem with low or dirty fluid is probably the most common cause of this error code. Therefore, by simply replacing the dirty fluid, the problem can be solved.

The fact that any anomaly in the transmission speed triggers gear problems causes the severity of the problem to vary.

In severe cases, the failure could trigger a major transmission problem, rendering the vehicle unable to shift gears. However, it could also be accompanied by other specific trouble codes.

Due to the complexity of the transmission problem, it is always best to have this code diagnosed and repaired by a qualified technician as soon as possible.

Components That Can Be Replaced Or Repaired With Code P0731

  1. Automatic Transmission Solenoid: Check for damaged transmission solenoids and replace damaged ones immediately. Transmission solenoids are a key factor in the operation of the transmission.
  2. Transmission control module: Sometimes the module itself may be faulty and cause interruptions in performance. In some cars, there is this TCM, exclusively for transmission control, while in others it is carried out directly by the PCM. In both cases, you should check their operation before deciding to change them.
  3. Automatic transmission sensor: It is important to note that the transmission sensor also plays a vital role in the operation of the TCM in relation to the PCM. If this sensor fails, the transmission will not perform its function.
  4. Automatic transmission torque converter: One of the most expensive failures you can have in your transmission. The torque converter may begin to fail due to the state of the ATF fluid. That’s why it’s very important.
  5. Automatic transmission connector: Defective or damaged will cause performance problems and therefore must be replaced in the first instance for the PCM and TCM to function properly.

Common Errors When Diagnosing Code P0731

The most common error when diagnosing the code P0731 It is a misdiagnosis of an engine misfire problem or a problem with the vehicle’s driveline.

If the system is not thoroughly inspected and diagnosed, it may also appear to be an internal transmission problem when in reality it is a much simpler and less expensive problem.

Is Code P0731 Serious?

Given the complexity of the failure and its possible consequences, we can consider this failure to have high severity. When this code occurs, our recommendation is that you take the car for diagnosis immediately.

As you may have noticed, the repair will range from simple to complex. You can solve it by changing the oil in some cases, while in others it will be necessary to disassemble the transmission and carry out internal repair.


You may also be interested in Error Code P0733: Incorrect Gear Ratio 3

Be sure to check the level and ATF fluid status before continuing with the diagnosis. Incorrect fluid levels or dirty fluid can cause shifting problems.

Transmission clutches are usually controlled by a solenoid or fluid pressure. If there is an electrical problem with the shift solenoid, a code related to this problem should be displayed.

Have the electrical problem corrected before continuing.

Clogged passages interfere with fluid circulation within the transmission, which can also cause error P0731.

If there are multiple gear ratio error codes, but the transmission is operating correctly, there may be a mechanical problem with the torque converter, main transmission control, or pressure issues.

If you want to know other articles similar to Code P0731. Incorrect Gear Ratio 1 you can visit the category Fault Codes.


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