P079B: Transmission Friction Element B Slip Detected

What does P079B mean?

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. 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, 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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

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

• High Clutch B plates worn or glazed
• Medium Too little line pressure: low or aged fluid, filter, auxiliary oil pump or blocked pressure orifice
• Medium Pressure or shift valve of clutch B or valve body sticking
• Low Module software (false setting)
• Low Faulty speed signals (motor resolver, output)

Likelihood is a general estimate across vehicles; your make and model may differ.