P0786

What it means

P0786 is the generic code for a range or performance fault with shift timing solenoid 'A' in an automatic transmission. The transmission control unit commands a gear change using the solenoid and sees that the actual shift timing, ratio change or pressure response does not match what it expected. The fault is not an outright electrical failure, so the cause is often mechanical or hydraulic as well as electrical.

Symptoms

• Delayed, harsh or flared gear changes
• Transmission stuck in one gear (limp mode)
• Gearbox warning light or check engine light on
• Slipping or engine revs rising between gears
• Poor acceleration or higher fuel consumption

Possible causes

• Low, degraded or contaminated transmission fluidlikely
• Shift timing solenoid sticking or worn, often from debris or varnish in the valve bodylikely
• Blocked transmission filter or restricted fluid passages reducing hydraulic pressurepossible
• Wiring or connector fault between the solenoid and transmission control unit, such as high resistance, corrosion or chafingpossible
• Worn valve body, internal leaks or worn clutch packs causing pressure losspossible
• Outdated or faulty transmission control unit software or a fault in the unit itselfrare

How to diagnose

1. Read all stored codes and freeze frame data, noting fluid temperature, speed, gear and when the fault set, and check for related codes
2. Check the transmission fluid level, condition and smell using the correct procedure for the model, as some are sealed and need a specific temperature and method
3. Inspect the wiring and connector at the transmission for damage, corrosion, fluid ingress or chafing
4. Check the solenoid circuit for resistance, continuity and short circuits against the manufacturer's specification
5. Use live data to compare commanded and actual gear, slip ratio and line pressure during a road test
6. Run solenoid activation tests with a suitable scan tool where supported to see whether it responds
7. Check for any manufacturer service bulletins or available software updates for the transmission control unit on that model
8. If the electrical side is sound, carry out a line pressure test and inspect the pan, filter and valve body for debris or wear

How to fix

• Correct the fluid level or replace degraded fluid and filter using the specified fluid type for the transmission
• Repair or replace damaged wiring or connectors at the transmission
• Replace the faulty shift timing solenoid, or the solenoid pack or valve body assembly if the design requires it
• Clean or overhaul the valve body and replace the filter if debris is blocking the passages
• Repair internal wear such as worn clutch packs or seals if pressure tests confirm it
• Update the transmission control unit software only if an outdated calibration is a known cause for that model

Can I keep driving?

Nearby codes

• P0785GenericShift Timing Solenoid A
• P0787GenericShift Timing Solenoid A Low