What it means
Friction element A (the first clutch or band in the gear matrix) shows measurable slip while it should be fully engaged. Detected by comparing input/output speed sensors against the commanded gear ratio. Slip means the element cannot transmit full torque — risk of glazing the friction surfaces.
At a glance
- Usually lights the check-engine light
How it works
Friction element A is a clutch or band inside the transmission that must hold firmly in certain gears. The TCM compares input and output speeds with the commanded gear ratio; P079A sets when the speeds show that element A is slipping while it should be applied.
Common causes
Most likely first.
- Worn friction discs in element A
- Low hydraulic pressure to element A
- Old ATF lacking friction modifier
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Review freeze-frame data and live input/output speed readings to identify the gear and conditions when the ratio indicated element A was slipping.
- Check transmission fluid level and condition using the vehicle's specified procedure, and inspect for leaks or signs of overheated fluid.
- Compare the slipping event with the commanded gear and shift data to confirm it occurs in a gear that uses friction element A.
- Inspect accessible transmission wiring and connectors if the data suggests an erroneous speed reading or control issue.
- If the speed data is valid and fluid is correct, test hydraulic pressure to the A element circuit and inspect for internal leakage or worn friction parts.
Common fixes
- Correct low fluid level or an external leak using the specified fluid and procedure.
- Service degraded fluid only if the vehicle's maintenance procedure calls for it.
- Repair a confirmed hydraulic restriction or leak affecting the element A circuit.
- Repair or replace worn friction element A components if internal inspection confirms damage.
Can I keep driving?
Avoid towing, hard acceleration, and continued driving while the transmission is slipping; friction element A can overheat and wear further. If it loses drive or cannot hold a gear, stop safely and arrange diagnosis.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.