P0731
PowertrainGear 1 Incorrect Ratio
This can be anything from a cheap fix to a gearbox out job, so don't panic until you've checked the easy stuff. The transmission control module times how fast the input shaft spins against the output shaft when you're in first. If those numbers don't add up to the ratio first gear is supposed to give, it logs P0731 and assumes the box is either slipping or not properly engaging the gear. On automatics that usually points at fluid, a solenoid, or worn internals.
ⓘ Information only. This page provides general educational information about fault code P0731. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0731 mean?
P0731 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Gear 1 Incorrect Ratio.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, sometimes the only thing you notice at first
- • Slipping when you pull away, revs climb but the car doesn't gather speed to match
- • Harsh or delayed thump when first gear takes up from a standstill
- • Car drops into limp mode and holds you in a higher gear, so pulling away feels gutless
- • Slow, laboured acceleration from stationary, like the handbrake is half on
- • Fuel economy creeps up because the box is working against itself
Possible causes
Causes commonly associated with P0731, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Low or burnt transmission fluid, the cheapest and most common reason. No hydraulic pressure means no clean shifts, and dark fluid that smells like toast tells you a lot
- 2. First gear shift solenoid sticking or failing electrically, so the right clutch pack or band isn't getting commanded properly
- 3. Worn internal friction material, clutch plates or bands past their best on a high mileage box. This is the expensive one
- 4. Input or output speed sensor giving rubbish readings, which makes the TCM think the ratio is wrong when the box is actually fine
- 5. Blocked or restricted fluid passages inside the valve body, often after years of skipped fluid changes
- 6. Wiring or connector trouble between the TCM and the solenoids or sensors, corroded pins and chafed looms throw these codes too
- 7. TCM software glitch or corrupted adaptation values, less common but worth ruling out before you condemn the gearbox
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Check the fluid level and condition properly. Many of these boxes want the level checked hot with the engine running, so do it by the book. Burnt smell or particles means internal wear is already on the cards
- 2. Scan for everything stored, not just P0731. Solenoid codes, speed sensor codes or a general P0700 alongside it point you in very different directions
- 3. Watch input and output speed sensor live data while driving or on a ramp. If the ratio reading is sensible but the code still sets, suspect the sensor signal rather than mechanical slip
- 4. Inspect the TCM and solenoid connectors for green corrosion, bent pins and water ingress. Wiggle the looms with the engine running and watch the live data for dropouts
- 5. Test the first gear solenoid resistance against spec with a multimeter where you can reach it, and check it's actually switching
- 6. If fluid, sensors and wiring all check out, run a line pressure test. Low pressure in first confirms the problem is inside the box
Common questions about P0731
Can I keep driving it or should I get it looked at straight away? +
Get it sorted soon. A slipping clutch pack generates heat and metal debris that contaminates the fluid and chews through the rest of the box, so every mile you drive while it's slipping makes the repair bigger and more expensive. If it's already gone into limp mode, drive it gently to a garage and stop running it like that. The exception is if it turns out to be just low fluid caught early, in which case topping up and clearing the code may settle it.
Is it the gearbox itself or could it just be a sensor or some dodgy wiring? +
Both are real possibilities and that's exactly why you check the cheap causes first. A speed sensor fooling the TCM or a corroded solenoid connector can throw P0731 with a perfectly healthy box, and that's a relatively small bill. But if the live data shows genuine slip and the fluid comes out dark and burnt, you're looking at worn internals and that's a different conversation entirely. Don't let anyone quote you for a rebuild before they've ruled out the fluid, sensors and wiring.
How long does a fix like this usually take? +
Depends massively on what it turns out to be. A fluid and filter service is a half day. A speed sensor or an externally mounted solenoid can be a couple of hours plus diagnosis. Once the gearbox has to come out for a rebuild or a recon unit, you're looking at a day or two of labour and often a wait on parts, especially on less common boxes. Budget low three figures for fluid or a sensor and well into four figures if the box needs opening up or replacing.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →