P0731: Gear 1 Incorrect Ratio (Meaning, Causes, Fixes)
What P0731 means
An automatic transmission uses several gear ratios, and the computer checks that each gear is actually working by comparing speed signals. RepairPal says P0731 is set when the powertrain control module (PCM) sees more than a specified RPM difference between the transmission's input speed sensor and its output speed sensor while in Gear 1, and that this often implies the transmission is slipping. OBD-Codes.com describes the check slightly differently: the ECM, PCM or transmission control module (TCM) often compares engine speed with the transmission speed sensor to work out the gear ratio and torque converter slip, and sets the code when the calculation doesn't match the desired value. RepairPal adds that the TCM uses solenoids to control the fluid pressure that applies each gear, and sets P0731 when it detects the transmission is not going into first gear. It is a generic OBD-II code for 1996 and newer vehicles, though repair steps vary by make and model. Related codes are P0730 (incorrect gear ratio) and P0732 to P0736 for the other gears; OBD-Codes.com says to repair any other transmission codes first.
Severity: can I keep driving?
Stop and fix promptly
Get it diagnosed soon and drive gently until you do. RepairPal rates P0731 Urgent, meaning it can cause severe damage or be unsafe, and says it often means the transmission is slipping. Both sources list shifting problems, and OBD-Codes.com adds that the car may not start out in 1st gear and may accelerate sluggishly. If the car won't move, slips badly, or pulls away weakly from a stop, avoid driving it in traffic and have the transmission checked.
Common symptoms
- Check engine light on (both sources)
- Vehicle won't shift properly, or shifts slip (RepairPal)
- Not starting out in 1st gear, sluggish acceleration (OBD-Codes.com)
- Transmission slipping (OBD-Codes.com; RepairPal says the code often implies slipping)
- Lower fuel economy (both sources)
- In unusual cases, no noticeable symptoms (RepairPal)
Likely causes
Roughly most to least common, based on the sources below. Your vehicle may differ.
- Low or dirty transmission fluid (OBD-Codes.com); RepairPal lists dirty fluid restricting the hydraulic passages
- Faulty shift solenoid (OBD-Codes.com); RepairPal names a defective Gear 1 shift solenoid
- Blocked passages or a problem in the valve body (main control) (both sources)
- Mechanical failure inside the transmission (OBD-Codes.com); RepairPal names a defective forward clutch
- Faulty transmission control module (OBD-Codes.com)
- Defective engine coolant temperature sensor (RepairPal)
- The sources differ: OBD-Codes.com lists the TCM and general internal failure, while RepairPal lists the forward clutch and the coolant temperature sensor and calls an internal transmission problem a common misdiagnosis
Diagnostic steps you can do
Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.
- Read all stored codes and write down the freeze frame data. If there are other transmission codes, such as a shift solenoid electrical code, OBD-Codes.com says to fix those first.
- Check the transmission fluid level and condition before anything else; both sources link low or dirty fluid to this code.
- Check for technical service bulletins (TSBs) for your vehicle; there may be a known fix.
- If only P0731 is stored, OBD-Codes.com suggests a careful test drive to see whether first gear actually engages. RepairPal says to note engine load, throttle position, RPM and road speed, because the code can be hard to duplicate.
Typical fixes
- Correct the fluid level, or service dirty transmission fluid
- Repair any shift solenoid electrical fault first, then replace a faulty Gear 1 shift solenoid if testing points to it
- Repair the valve body (main control) if passages are blocked or pressure is wrong
- Repair internal parts such as the forward clutch when diagnosis confirms mechanical failure
- Replace or reprogram the TCM only after other causes are ruled out
Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.
🛠 When to call a pro
- The car won't move in Drive, slips, or won't start out in first gear
- Several incorrect-ratio codes are stored at once; OBD-Codes.com says that points to internal mechanical, pressure or blocked-passage problems
- Diagnosis needs an advanced scan tool to see the commanded gear and compare speeds; OBD-Codes.com says this repair often requires advanced transmission knowledge and the factory service manual
- Fluid checks out and the code keeps coming back
Where to buy
Plain search links. Always match parts to your exact year, make, model and engine.
FAQs
Is P0731 serious?
Yes, treat it as serious. RepairPal rates it Urgent and says it often means the transmission is slipping. Drive gently and get it diagnosed soon to avoid further transmission damage.
Can low transmission fluid cause P0731?
It can. OBD-Codes.com lists low or dirty fluid as a cause and says to check fluid level and condition first; RepairPal lists dirty fluid restricting the hydraulic passages.
Does P0731 mean I need a new transmission?
Not necessarily. Both sources list fixable causes such as fluid problems, a shift solenoid or the valve body. RepairPal even lists an internal transmission problem as a common misdiagnosis, so get a proper diagnosis before agreeing to a rebuild.
How much does it cost to fix P0731?
Neither source gives a repair cost. The price ranges widely depending on whether it's fluid, a solenoid, the valve body or internal parts, so get a diagnosis and a written estimate first.
Sources
Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.