Low or degraded transmission fluid, or the wrong fluid type; Worn clutch pack or band for fourth gear, or a worn torque converter clutch; Faulty shift or pressure control solenoid, or a sticking valve body valve; Faulty input or output speed sensor, or a wiring or connector fault in its circuit; Low line pressure from a worn pump, internal seal leaks or a blocked filter; Outdated transmission control module software (some models have a known fault); Wrong tyre size or a faulty wheel speed signal causing a false ratio calculation.

P0734

Gear 4 Incorrect Ratio.

Generic
Powertrain
Generic code
High severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
7: AreaTransmission
34: FaultThe specific fault in that area

What it means

P0734 means the transmission control module has detected that the actual ratio in fourth gear does not match the expected ratio. It calculates this by comparing input shaft speed with output shaft speed, and the result shows slip or an incorrect ratio while fourth gear is commanded. It applies to automatic and some automated transmissions, and the exact cause depends on the model and gearbox type.

Symptoms

Engine revs rise without matching acceleration in fourth gear (slipping)
Flare or delayed engagement when shifting into or out of fourth
Harsh or erratic shifts at higher speeds
Gearbox stuck in limp mode or a fixed gear
Gearbox warning light or engine light on
Poor fuel economy and sluggish performance on the motorway

Possible causes

Low or degraded transmission fluid, or the wrong fluid typelikely
Worn clutch pack or band for fourth gear, or a worn torque converter clutchlikely
Faulty shift or pressure control solenoid, or a sticking valve body valvelikely
Faulty input or output speed sensor, or a wiring or connector fault in its circuitpossible
Low line pressure from a worn pump, internal seal leaks or a blocked filterpossible
Outdated transmission control module software (some models have a known fault)rare
Wrong tyre size or a faulty wheel speed signal causing a false ratio calculationrare

How to diagnose

Read all stored and pending codes plus freeze frame data, noting speed, load, gear and fluid temperature when the code set
Check the transmission fluid level, condition and smell using the correct procedure for the model, as some are sealed or need a set temperature
Inspect wiring, connectors and the transmission harness for damage, corrosion, or fluid ingress
Compare live input speed, output speed and calculated gear ratio against specification while road testing, looking for slip in fourth
Check live data for the shift solenoid duty cycle and line pressure, where the transmission supports it
Test speed sensor circuits and signals, including resistance and waveform according to the manufacturer data
Check the tyre sizes match and that wheel speed data is plausible
Check for software updates or technical bulletins for the model, then pressure test or inspect the valve body and internals if the electrical checks pass

How to fix

Correct the fluid level or replace degraded fluid and filter with the specified type
Repair or replace damaged wiring, connectors or faulty speed sensors
Replace faulty solenoids or clean, repair or replace the valve body
Overhaul or replace the transmission or torque converter if clutches or pump are worn
Update the transmission control module software if an outdated calibration is a known cause for that vehicle
Fit the correct tyre sizes or repair the wheel speed signal fault if it caused a false ratio reading

Can I keep driving?

High severity

Limit driving, as continued slip can quickly overheat and wear the clutches and lead to a costly failure. Drive only as far as needed to get it inspected, and avoid heavy loads, towing and sustained high speeds.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.