What it means
Same condition as element A slip, applied to friction element B (the second clutch or band).
At a glance
- Usually lights the check-engine light
How it works
Friction element B is a transmission clutch or band that holds specific internal components when the TCM commands gears that use it. P079B sets when input and output speed readings show that element B is slipping instead of holding the expected gear ratio.
Common causes
Most likely first.
- Worn or damaged friction element B
- Low or degraded transmission fluid
- Hydraulic pressure loss or internal leakage
- Valve body or control fault affecting element application
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 speeds to see which gear and conditions show the ratio slip associated with element B.
- Check the transmission fluid level and condition by the vehicle's specified procedure, and inspect for leaks or fluid degradation.
- Compare the event with commanded gear data to confirm that slip occurs in a gear that applies friction element B.
- Check transmission connectors and speed sensor data for faults that could make the calculated ratio misleading.
- If the ratio readings are reliable, check hydraulic pressure and valve body operation in the element B circuit, then assess for internal leakage or worn parts.
Common fixes
- Correct low fluid level or repair an external leak using the vehicle's specified procedure.
- Service degraded fluid if required by the vehicle's maintenance instructions.
- Repair a confirmed valve body fault or hydraulic pressure loss affecting element B.
- Repair or replace worn friction element B components if internal inspection confirms damage.
Can I keep driving?
Avoid towing, hard acceleration, and driving that causes repeated slip; element B can overheat and internal wear can progress. Stop if the transmission loses drive or cannot hold a gear reliably.
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.