P0731
Gear 1 Incorrect Ratio
Get Your FREE AI Diagnostic Report
Enter your vehicle for a personalized diagnosis for code P0731
The P0731 code means the Powertrain Control Module (PCM) has detected that the actual gear ratio in first gear does not match the expected ratio. The PCM monitors the difference between the transmission input shaft speed and the output shaft speed. When the vehicle is supposed to be in first gear but the ratio between these two speeds is outside the normal range, this code is set.
This is a significant transmission concern that should not be ignored. Driving with this code active can cause further damage to internal transmission components, turning a potentially simple repair into an expensive transmission rebuild. The most common culprit is low or degraded transmission fluid, so checking your fluid level and condition is the best first step.
If the fluid is dark, burnt-smelling, or low, a fluid service may resolve the issue. However, if the fluid looks clean and is at the proper level, the problem likely lies with a shift solenoid, speed sensor, or internal transmission wear. Because transmission diagnosis requires specialized knowledge and tools, most drivers should have this code diagnosed by a qualified transmission specialist rather than attempting DIY repairs beyond a basic fluid check.
Severity
Symptoms
- •Harsh or delayed shift into first gear
- •Transmission slipping when starting from a stop
- •Check Engine light illuminated
- •Vehicle feels sluggish or hesitant when accelerating from rest
- •RPMs flare high without corresponding acceleration
Likely Causes
Insufficient or contaminated ATF causes incorrect hydraulic pressure in first gear, leading to slippage and an incorrect ratio reading between input and output speed sensors.
The shift solenoid responsible for engaging first gear may be stuck, worn, or electrically faulty, preventing proper gear engagement and causing a ratio mismatch.
Worn clutch packs, bands, or planetary gear sets inside the transmission can cause slippage in first gear, resulting in the PCM detecting an incorrect gear ratio.
A failing transmission speed sensor may send inaccurate RPM data to the PCM, causing it to calculate an incorrect gear ratio even if the transmission is functioning properly.
A worn or sticking valve body can misdirect hydraulic pressure, preventing first gear from fully engaging and causing the ratio discrepancy.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Check and top off or replace transmission fluid
- Replace the faulty shift solenoid
- Replace the input or output speed sensor
- Repair or replace the valve body
- Rebuild or replace the transmission if internal damage is found
Shop Parts
Videos
Causes and Fixes P0731 Code: Gear 1 Incorrect Ratio
HVAC Mechanic
💛 P0731 Code: CAUSES and SOLUTIONS【2025】💛
DTCcodes
What is P0731 : Engine Error Code Causes Explained
Auto User Guide