Gear 1 Incorrect Ratio
The Powertrain Control Module (PCM) or Transmission Control Module (TCM) detected that the actual gear ratio for first gear does not match the expected gear ratio. This is determined by comparing the input (turbine) speed sensor signal with the output speed sensor signal while the transmission is commanded into first gear.
A first gear incorrect ratio can cause severe drivability issues, including the inability to accelerate from a stop. Continued driving can cause further internal transmission damage or leave you stranded.
Common symptoms
- Check Engine Light illuminated
- Transmission slipping or hesitating when accelerating from a stop
- Vehicle fails to move forward in Drive (loss of first gear)
- Transmission shifts harshly into or out of first gear
- Vehicle goes into Limp Mode (stuck in a default gear)
Likely causes
- Low or dirty transmission fluid
- Defective shift solenoid responsible for first gear
- Faulty input or output transmission speed sensors
- Damaged wiring or poor connections in the transmission harness
- Internal transmission failure (worn clutches, bands, or valve body issues)
- Faulty Transmission Control Module (TCM)
What to check before replacing parts
Check the transmission fluid level and condition
Look for: Fluid that is low, burnt, or heavily contaminated with metal shavings, which indicates a leak or internal mechanical wear.
Scan for related DTCs and monitor live data
Look for: Input and output speed sensor RPM readings that do not match the commanded first gear ratio, or the presence of other solenoid/sensor codes.
Inspect the transmission wiring harness and connectors
Look for: Corroded pins, frayed wires, or loose connectors at the transmission case, speed sensors, and TCM.
Test the transmission input and output speed sensors
Look for: Resistance or voltage signals that fall outside manufacturer specifications, or erratic readings when viewed on a scan tool or oscilloscope.
Perform a transmission line pressure test
Look for: Hydraulic pressures that are lower than manufacturer specifications in first gear, indicating a failing pump, valve body issue, or internal fluid leak.
This page explains the generic P0731 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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