What P079B means
The 'friction element slip' codes monitor a clutch that is commanded locked in a power-split hybrid transmission: the module calculates its slip from electric motor, engine and output speeds (target 0 rpm) and flags slip above a threshold for a minimum time, with sufficient line pressure and clutch torque; from 2016 GM also flags a calculated friction energy threshold at once.
How the engine computer detects it
The letter matches the manufacturer's clutch number. P079B concerns clutch 2. GM's Two-Mode flags above 200 rpm for 1 s, after two trips; the 2013 Volt above 200 rpm after one trip; Malibu Hybrid and Volt from 2016 100 rpm for 2.5 s at more than 15 Nm (11 lb-ft), or 5851 J of friction energy, after one trip. Chrysler's Pacifica PHEV lists it as friction element 2 with MIL and a permanent code.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light or 'service hybrid system' message
- Limp mode with reduced power or restricted drive mode
- Slipping, engine speed rises without drive
- Jolt when the engine starts or stops
Common causes
- Clutch B plates worn or glazed Most likely
- Too little line pressure: low or aged fluid, filter, auxiliary oil pump or blocked pressure orifice Possible
- Pressure or shift valve of clutch B or valve body sticking Possible
- Module software (false setting) Less likely
- Faulty speed signals (motor resolver, output) Less likely
How to diagnose P079B
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: transmission clutches, transmission valve body, auxiliary transmission oil pump, hybrid control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 7
- Transmission
- 9B
- Specific fault
Questions drivers ask
Can I drive with code P079B?
If the car shifts normally, drive gently to a shop. If it's stuck in one gear, slips, shifts harshly or won't move out of Park, don't drive it: have it towed.
Will my car pass an emissions inspection with P079B?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Will P079B clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.