P0731 OBD-II: Gear 1 Incorrect Ratio

What Does P0731 Code Mean?

OBD-II Code P0731 is defined as a Gear 1 Incorrect Ratio

The purpose of the automatic transmission is to match the engine's optimum power and torque characteristics to the driver's desired rate of acceleration and speed by auto-selecting different gear ratios or 'speeds' to power the wheels.

Code P0731 When the code P0731 is set in the Powertrain Computer, it means that the Powertrain Computer or PCM is seeing greater than a specified RPM difference between the rotational speed of the Input RPM Sensor and the Transmission Output RPM Sensor in Gear 1. This can occur during shifting or when driving at steady speeds in the same gear. It often implies that the transmission is slipping.

P0731 Symptoms

• Check Engine Light will illuminate
• Vehicle will not shift properly
• Decrease in fuel economy
• In unusual cases, there are no adverse conditions noticed by the driver
• In some cases, there may be performance problems, such as improper/slipping gear shifts

Common Problems That Trigger the P0731 Code

• Defective Gear 1 Shift Solenoid
• Defective Forward Clutch
• Defective Engine Coolant Temperature Sensor
• Defective Valve Body
• Dirty transmission fluid that restricts the hydraulic passages

Common Misdiagnoses

• Engine Misfire problem
• Internal Transmission problem
• Driveline problem

Polluting Gases Expelled

• HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
• CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
• NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog

P0731 Diagnostic Theory for Shops and Technicians

When diagnosing a P0731 code, it is important to record the freeze frame information and then to duplicate the code setting conditions with a test drive. Pay close attention to the engine load, throttle position, RPM, and road speed because a P0731 can be difficult to detect.