P0731
Generic (SAE)
Gear 1 Incorrect Ratio
Severity
High
Typical repair cost
150 – 3 000 EUR
System
Powertrain
What it means
With 1st gear commanded, the TCM expects input speed divided by output speed to equal the 1st gear ratio within a few percent. P0731 sets when the actual ratio is too high (slip) or too low (wrong gear engaged) for a defined time. Because 1st gear carries the highest torque, slip here usually means a worn forward clutch, low-reverse brake or a leaking circuit in the valve body.
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Symptoms
- Engine revs high when pulling away but the car creeps or hesitates
- Delayed engagement of Drive from standstill, especially cold
- Harsh 1-2 shift or the transmission starts off in 2nd (limp mode)
- Burnt fluid smell after stop-and-go traffic
- MIL on, gear indicator may flash
Common causes
- Worn forward clutch or low/reverse brake friction plates
- Low fluid level or aerated fluid starving the 1st gear circuit
- Leaking piston seals, sealing rings or clutch drum cracks
- Sticking shift valve or worn bore in the valve body for the 1st gear circuit
- Failed one-way (sprag) clutch in the planetary set
- Input or output speed sensor giving a faulty reading at low speed
How to diagnose
- Check fluid level at the correct temperature and inspect colour and smell; black or burnt fluid indicates overheated friction material.
- Road test while graphing input and output speed in 1st gear; calculate ratio = input/output and compare with spec (e.g. 4.17:1). Slip above 5 % at 2500 rpm input (more than about 125 rpm excess) confirms internal slip.
- Perform a stall test briefly (3-5 s) in D with brakes applied: stall RPM 300-500 rpm above spec means a slipping clutch or one-way clutch; 300 rpm below spec means a weak converter stator.
- Measure line pressure in D at idle (typically 3.5-5 bar) and at stall (12-18 bar); low pressure in D only points to the forward clutch circuit or sealing rings.
- Drop the pan and inspect the filter for clutch fibre and metal; check the 1st gear solenoid resistance (10-25 ohms for on/off types) and actuation.
- Decide between valve body/mechatronic repair (clean pan, low pressure) and overhaul or remanufactured transmission (debris, slip in multiple gears).
Frequently asked questions
Why does the transmission start in 2nd gear now?
That is the limp strategy. The TCM avoids the slipping 1st gear circuit and uses a gear it can hold, trading acceleration for protection.
Could a solenoid alone cause P0731?
Yes, if the 1st gear control solenoid sticks or leaks, pressure to the clutch drops and it slips. Solenoid replacement is far cheaper than an overhaul, so test it first.