Car systems
Transmission: how it works and what fails
The transmission carries the engine's rotation to the wheels: the clutch or the torque converter couples it, the gearbox picks the ratio and the axle shafts and differential share it out. Automatic transmissions have their own control module, which stores its codes and, when it detects a problem, holds the transmission in a limp-home gear.
Reviewed by Daniel Morales, founder of Reprorace SL, on
How it works
In a manual transmission, the clutch separates the engine from the gearbox while you pick a gear. An automatic uses a torque converter or its own clutches and shifts by itself with hydraulic pressure controlled by solenoid valves; dual-clutch units preselect the next gear on a second shaft.
The transmission control module (TCM) reads the input and output speed sensors, the fluid temperature and the shifter position. If the measured ratio does not match the gear engaged, it stores a code (P0730 and following) and may hold a fixed gear to protect the transmission. All-wheel drive adds a transfer case or a coupling with its own control.
The most common faults
- Worn clutch: it slips: the engine revs up without the car speeding up, especially in high gears.
- Automatic transmission fluid: old or low: harsh shifts, delays and overheating.
- Solenoids and valve body: shifts at the wrong time or with a clunk, and codes from P0750 on.
- Transmission speed sensors: the module loses its reference and goes into limp mode.
- CV joints and mounts: clunks when you accelerate or lift off, vibration and clicking when turning.
A guide, most likely first: a repair shop confirms the diagnosis of each car with its own data.
Symptoms that can come from this system
Automatic gearbox jerks: causes and what to do
See a garage soon
Slipping clutch: symptoms, causes and what to do
See a garage soon
Gears hard to engage or grinding: causes and what to do
Check without hurry
Car vibrates when accelerating: what it could be
See a garage soon
Automatic transmission warning light on: what it means
See a garage soon
Car loses power: what limp mode is
See a garage soon
Noise when turning the steering wheel: what a click or a creak means
See a garage soon
The most relevant trouble codes
- P0700Transmission control system: MIL illumination requested
- P0705Transmission range sensor A, circuit: PRNDL input
- P0715Input/turbine shaft speed sensor A, circuit: malfunction
- P0720Output shaft speed sensor, circuit: malfunction
- P0730Incorrect gear ratio
- P0731Gear 1: incorrect ratio
- P0732Gear 2: incorrect ratio
- P0741Torque converter clutch, circuit: performance or stuck off
- P0750Shift solenoid A: malfunction
- P0841Transmission fluid pressure sensor/switch A, circuit: range/performance
- P0868Transmission fluid pressure: too low
- P0882TCM power input signal: signal low
- C00A1Driveline control module: malfunction
Generic OBD-II/EOBD codes; each manufacturer adds its own. The same symptom can come from another system.
Code blocks of this system
Questions and articles about this system
- What does a pending, stored or permanent code mean?
- What is live data and what is it for?
- What is freeze frame data?
Learn more
See what your car is saying, system by system
The ECO-LITE plugs into the OBD port and shows the codes and live data your car publishes on your phone, explained; when needed, you share the session with your repair shop.
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Frequently asked questions
What is the transmission's limp mode?
It is a fixed gear, often third, that the module uses to protect the transmission when it detects a fault. It lets you drive calmly to a repair shop, but you should not drive like that for days.
Does automatic transmission fluid need changing?
Many manufacturers call it lifetime fluid, but changing it at service is common, especially with hard use. Follow your model's schedule.
Can an OBD reader see the automatic transmission?
It sees the generic transmission codes (P07xx to P09xx) reported over OBD. The manufacturer's own codes and the transmission fluid temperature are sometimes only available with its tool.