P0779

What it means

P0779 is the generic code for an intermittent fault in the Pressure Control Solenoid 'B' circuit or its performance in the automatic transmission. The transmission control unit saw the solenoid circuit or the pressure it controls drop in and out of the expected range, rather than fail constantly. Which solenoid is 'B' and what it controls depends on the make, model and gearbox, so check the vehicle-specific data.

Symptoms

• Harsh, delayed or flared gear changes that come and go
• Gearbox going into limp mode or stuck in one gear, sometimes only intermittently
• Transmission warning light or engine warning light on
• Slipping under load or when the fluid is hot
• Loss of lock-up or erratic shift feel

Possible causes

• Wiring or connector fault in the solenoid circuit (chafing, corrosion, loose pin, internal harness fault)likely
• Worn or sticking pressure control solenoid B with intermittent internal open circuit or high resistancelikely
• Low, degraded or contaminated transmission fluid, or a blocked filter, causing unstable pressurelikely
• Valve body wear or debris causing the solenoid or its valve to stick intermittentlypossible
• Low or unstable battery or charging system voltage affecting solenoid controlpossible
• Fault inside the transmission control unit driver circuit, or an outdated calibration where the manufacturer has issued a fixrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, including fluid temperature, speed and load when the fault set
2. Check the transmission fluid level and condition, following the manufacturer's procedure for the correct temperature and method, and look for burnt smell, debris or leaks
3. Check battery condition and charging voltage, as low voltage can cause intermittent solenoid faults
4. Inspect the gearbox wiring harness and connectors for chafing, oil ingress, corrosion and bent or loose pins, and wiggle test with live data or a circuit monitor
5. Measure the solenoid resistance and compare it with the manufacturer's specification, checking circuit continuity and shorts to earth or supply
6. Watch live data for commanded versus actual pressure or gear ratio during a road test to see when the fault appears
7. Check for manufacturer service bulletins or calibration updates relevant to the model and gearbox
8. If the electrical side is sound, inspect the valve body and solenoid for sticking or debris, as per the model's procedure

How to fix

• Repair or replace damaged wiring, terminals or connectors, including an oil-contaminated internal harness where fitted
• Replace the faulty pressure control solenoid B, or the solenoid pack, as the model requires
• Correct the fluid level or replace fluid and filter with the specified type if it is degraded or contaminated
• Clean or overhaul the valve body, or replace it if worn or damaged
• Repair charging or battery faults to restore stable voltage
• Update the transmission control unit calibration or replace the unit only if testing proves it faulty or a known calibration issue applies

Can I keep driving?

Nearby codes

• P0778GenericPressure Control Solenoid B Electrical
• P077AGenericOutput Shaft Speed Sensor Circuit - Loss of Direction Signal