P0784

What it means

P0784 is the generic SAE J2012 code for a 4-5 shift malfunction. The transmission control unit commanded a shift between fourth and fifth gear (up or down) and saw that the gear ratio, input/output speed relationship or shift timing did not match what was expected. It usually means the shift did not complete correctly, so it points to a mechanical, hydraulic or electrical fault in the transmission.

Symptoms

• Harsh, delayed or flaring shift between fourth and fifth gear
• Transmission slipping or revving up when changing to or from fifth
• Gearbox stuck in fourth or dropping into limp mode
• Gearbox warning light or check engine light on
• Poor fuel economy or high revs at cruising speed
• Failure to select fifth gear on the motorway

Possible causes

• Low, degraded or contaminated transmission fluidlikely
• Faulty or sticking shift solenoid or pressure control solenoid involved in the 4-5 shiftlikely
• Worn or sticking valve body, or internal hydraulic pressure leakpossible
• Worn clutch packs, bands or seals for the gear or clutch used in the 4-5 shiftpossible
• Wiring or connector fault to the solenoids or speed sensors, including internal harness damagepossible
• Faulty input or output speed sensor giving incorrect ratio readingspossible
• Faulty transmission control unit or outdated software where a known fault is documented for the modelrare

How to diagnose

1. Read all stored codes and freeze frame data, noting speed, load, gear and fluid temperature when the fault set, and check for related codes
2. Check the transmission fluid level, condition and smell, following the manufacturer's procedure for that gearbox as some are sealed or need a set temperature
3. Inspect wiring, connectors and the transmission harness for damage, corrosion, fluid ingress or chafing
4. Check for any technical service bulletins or software updates for the model and gearbox
5. Use live data to compare commanded gear with actual ratio, and check input and output speed sensor readings for plausibility
6. Test the solenoid circuits and resistance against specification for the specific gearbox
7. Carry out a line pressure test or road test to see if the slip occurs on the 4-5 shift only; the procedure depends on the model
8. If the fault remains, inspect the valve body and clutch components, which usually needs the pan or gearbox removing

How to fix

• Correct the fluid level or replace degraded fluid and filter using the specified type
• Replace the faulty shift or pressure control solenoid
• Repair or replace damaged wiring, connectors or speed sensors
• Clean, repair or replace the valve body if it is worn or sticking
• Overhaul or replace the gearbox if clutches, bands or seals are worn
• Update or replace the transmission control unit software or unit only where a known fault is confirmed

Can I keep driving?

Nearby codes

• P0783Generic3-4 Shift
• P0785GenericShift Timing Solenoid A