P079B

Code

Generic P — Powertrain

Transmission Friction Element B Slip Detected

Brand: Generic

Type: P — Powertrain

Views: UK: 23 EN: 29 RU: 22

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Low or contaminated automatic transmission fluid (ATF) level
Burnt or degraded ATF (loss of friction properties)
Worn or glazed clutch plates or bands in Friction Element B
Hydraulic pressure loss to the element (leaks, worn bore, damaged seals)
Stuck or faulty shift/apply solenoid or control valve
Valve body internal wear or contamination

Symptoms

Transmission slips or slips just after engagement in a specific gear range
Delayed or soft engagement into drive or the affected gear
Harsh, delayed, or erratic shifts involving element B
Loss of power to wheels under load, poor acceleration
Check Engine Light / transmission MIL illuminated
Possible transmission overheating or burning smell

What to check

Read and record P079B plus any other transmission/engine codes and freeze-frame data
Note conditions when code set (gear, engine RPM, load, temperature)
Check ATF level and inspect fluid color/odor (burnt smell, dark color, debris)
Visually inspect for external fluid leaks and wiring/connectors to transmission solenoids
Road-test with data logging to reproduce slip and capture parameters (speeds, commanded apply, pressures)
Monitor TCM commanded duty/current and actual feedback for the solenoid(s) or clutch apply circuit

Signal parameters

ATF temperature
Transmission line pressure (kPa/psi)
Clutch apply pressure for Friction Element B (kPa/psi)
Commanded duty cycle/current for the solenoid that controls Element B
Solenoid status (ON/OFF) and response time
Engine rpm vs transmission input/turbine and output speeds (slip % = input vs output difference)

Diagnostic algorithm

Capture freeze-frame and live data to confirm conditions when P079B set; reproduce by road-test if safe.
Verify ATF level, condition and correct fluid type; correct level and replace if contaminated or burnt (perform fluid and filter service if needed).
Clear codes and retest after fluid service — if code returns, continue diagnosis.
While logging, confirm TCM is commanding correct apply and that commanded duty/current matches expected. Monitor apply pressure and input/output speeds for slip when command is active.
Inspect wiring and connectors for the solenoid(s) or pressure control valves for open, short, corrosion, or poor ground; check resistance of solenoid coil against spec.
Perform solenoid functional test (active tests) and pressure tests: measure direct line/apply pressure with a gauge while element commanded. Compare to specification. Low or no pressure indicates hydraulic/mechanical issue.
If electrical and external hydraulic systems check good but apply pressure is low, remove valve body for inspection for stuck valves, debris, or bore wear; inspect separator plate and check valve clearances.
If valve body and solenoids are OK, consider internal wear — inspect clutch pack, piston, seals, servo and drum for wear or burnt friction material; rebuild or replace as necessary.
Repair or replace failed components (solenoid, valve body, seals, clutch pack or transmission assembly) as indicated. Update TCM software/calibration if applicable.
After repair, clear codes, perform relearn/adaptation procedures per manufacturer, retest road conditions to verify code does not return.

Likely causes

Low ATF level or incorrect ATF type
Contaminated/burnt fluid reducing friction
Worn clutch pack or friction material for element B
Failed or sticking solenoid controlling element B
Hydraulic leak or bore/seal wear reducing apply pressure

Fault status

⚠️ Status

TCM detected slip of Transmission Friction Element B (P079B). Drive may be reduced, shifting abnormal, or vehicle may enter limp mode. Further diagnosis required.

🔴 Repair difficulty: Hard

⏱️ Diagnostic time: 2.0-6.0 hours