P0731
Gear 1 Incorrect Ratio
If your code reader dashboard displays the generic DTC error code P0731, your vehicle is currently experiencing an active failure related to "Gear 1 Incorrect Ratio". Proper system troubleshooting is required to safely clear this warning.
Driver's Summary
The diagnostic trouble code P0731 indicates an active fault in the gear 1 incorrect ratio circuit or component. In practice, this fault causes transmission slips in 1st gear, harsh shift into 2nd, slow acceleration from stop. This is a serious fault — avoid extended driving and have your vehicle inspected by a mechanic as soon as possible to prevent further damage.
Symptoms
Transmission slips in 1st gear, harsh shift into 2nd, slow acceleration from stop
Common Causes
- Low transmission fluid
- Defective 1st gear shift solenoid
- Internal mechanical failure (worn 1st gear clutch)
- Clogged transmission filter
How to Fix
- 1 Top off transmission fluid
- 2 Replace shift solenoid for 1st gear
- 3 Change transmission fluid and filter
- 4 Overhaul transmission
Technical Explanation
The ECM detects code P0731 by continuously monitoring the relevant sensor circuit against calibrated threshold values stored in its non-volatile memory. The TCM compares the ratio between input turbine speed sensor and output speed sensor readings against the expected gear ratio stored for each commanded gear position. A deviation greater than a few percent indicates clutch slippage, solenoid malfunction, or internal mechanical failure. The MIL illuminates after the fault is confirmed on two consecutive drive cycles, and the freeze frame data captured at first detection is stored in the PCM's memory for diagnostic reference.
Is It Safe to Drive?
Driving with an active P0731 fault risks accelerating damage to low transmission fluid and related components. The longer the fault persists, the more expensive the eventual repair becomes — what starts as a sensor or solenoid issue can escalate to major mechanical failure.
Mechanic's Pro Tip
For P0731, test the solenoid's coil resistance with a multimeter before ordering parts — most solenoids should read between 14 and 40 ohms; an open (infinite resistance) or short (near zero) confirms it's failed electrically. Also verify the PCM is commanding the solenoid by backprobing the connector with a test light during the relevant operating condition — if there's no command signal, the fault is in the PCM or wiring, not the solenoid itself.
Solenoid and fluid: $250 - $500; Transmission overhaul: $2,000+