Table of Contents
Error Code P0731 is described as Gear 1 Incorrect Ratio. This means there’s a problem with the automatic transmission on the first gear, usually caused by problems with solenoid or dirty transmission fluid.
This code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II system, particularly those made since 1996 up to present. Specifications on the definition, troubleshooting and repairs of course, vary from one make and/or model to another.
Modern vehicles that come equipped with automatic transmission or tranaxles, use torque converter between the engine and transmission to improve the torque output of the engine and drive the rear wheels. The PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes), controls automatic transmission by using multiple gear ratio and increase the vehicle speed to maximize the engine’s power output. Newer vehicle models are equipped with more than four gear ratios for fuel efficiency.
The PCM decides when to shift up or down between gears, based on the vehicle speed vs. throttle position. It does so by using the input coming from various sensors to verify the correct operation of the transmission and all its components.
Error Code P0731 happens when there’s a problem in shifting into 6th gear or while driving into first gear.
Other related codes include:
Error codes P0729, P0730, P0732, P0733, P0734, P0735 and P0736.
If there are other transmission code problems, they should be fixed first before fixing the incorrect fear ratio code.
As with other error codes P0731 triggers the Check Engine light. Common symptoms of this error code may include the following:
- Delayed shifting or vehicle not shifting into the right gear
- Sluggish acceleration
- Not starting in 1st gear
- Slipping transmission
- Decrease in fuel efficiency
Error Code P0731 can be caused by multiple factors, such as:
- Insufficient level or dirty transmission fluid
- Mechanical failure inside transmission
- Clogged fluid filter or worn pump
- Internal blockage in transmission main control
- Solenoid, torque converter clutch or internal blockage
- Faulty shift wirings or solenoids
- Faulty transmission control module
- Defective torque converter clutch
How to Check
Before starting the diagnosis, mechanics would first check the transmission fluid’s level and quality. Old and dirty fluid must be replaced.
To diagnose this error code, technicians use their OBD-II scanner to view real-time information. The codes will then be cleared and the vehicle will be taken for a test drive. If the code comes back, then the problem is really caused by an actual failure in the transmission.
The bands and internal clutches of the transmission are usually controlled by the solenoid or through fluid pressure. Any electrical issue with a shift solenoid can cause the error code. Thus, it is important to fix electrical problems before proceeding to the next step of diagnosis.
Also, a blocked fluid passage inside the transmission can keep the vehicle from starting out at 1st gear. If there are multiple related codes, but the transmission performs as expected, the problem could be a mechanical issue within the torque converter, a pressure problem or a transmission main control problem.
If there are multiple incorrect ratio codes and shift problems, but not all are displayed, the fault can be a mechanical problem internal to 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 stall speed of the engine is within the factory specifications or not. The engine must be able to match the readings from the transmission sensor.
How to Fix
Depending on the result of the diagnosis, the repairs for this error code are as follows:
- Check all electrical connections and wires that connects the sensors to the transmission control module
- Repair or replace damaged or corroded wires in and around the transmission control module
- If transmission fluid and wiring are both in good shape, transmission components must be checked for issues and repaired, or replaced if necessary
- If there are problems with the shift solenoids, pump, or torque converter, the transmission must be removed for proper repairs of these components
- After conducting the necessary repairs, the transmission must be replaced and checked before being tested to make sure everything works properly without the trouble codes, including P0730.
Low or dirty transmission problem is probably the most common cause of this error code. Thus, by simply replacing the dirty fluid, the issue can be resolved.
The fact that any abnormality in the transmission speed will trigger problems to the gear, the severity of the problem can vary. In severe cases, the problem could trigger major transmission issue, rendering the vehicle unable to change gear. However, it could also be accompanied by other specific trouble codes.
Because of 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.