P079A: Transmission Friction Element A Slip Detected
What to do
Get this looked at soon, and drive gently until you do. A slipping gearbox destroys its own fluid with the heat it makes, and the degraded fluid then causes more slip — so this is one of the few faults that genuinely gets worse quickly. Caught early it is often fluid and a solenoid; left alone it becomes a rebuild.
Say when the fluid was last changed, and whether the correct specification was used. Transmission fluid is not generic — it is the friction material as much as the hydraulic fluid, and the wrong one causes slipping in a mechanically sound gearbox.
Be wary of a shop that recommends only a fluid change without checking pressures, and equally wary of one that quotes a rebuild without checking the fluid first.
Mention any flaring — the engine revs rising during a gearchange without the car accelerating. That is slip you can feel.
What this code means
The first friction element is slipping under load rather than holding.
A friction element is the general name for the clutch packs and brake bands that actually select gears inside an automatic gearbox. A gear is not engaged by moving anything into mesh — it is engaged by clamping the right combination of elements, so every gear is a particular set of them held tight while the rest are released.
Slip means one is not holding. The module detects it by arithmetic: input speed divided by output speed should give the ratio of the commanded gear, and an element letting go makes that number wander while the gear is still commanded.
What matters most about this code is that the fault accelerates itself, and saying so is the useful part.
A slipping element generates heat. Heat destroys the fluid. Degraded fluid loses the pressure and the friction properties the element needs. So it slips more. A gearbox that is caught slipping early is often repairable by fluid and a solenoid; the same gearbox six months later is a rebuild.
The causes, cheapest first:
- Fluid low, old or the wrong specification. The fluid is both the hydraulic medium and the friction modifier — the wrong one causes slip on a mechanically perfect gearbox.
- Insufficient apply pressure, from a pressure control solenoid or a tired pump.
- Worn friction material, which is the genuine internal wear.
Likely causes
Start here
- Low or degraded transmission fluid · moderate
- Worn transmission friction element · specialist
- Faulty pressure control solenoid · specialist
Less common
- Worn valve body · specialist
- Worn transmission fluid pump · specialist
Symptoms
Components involved
Information, not instruction
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Community experience
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