P0789

What it means

P0789 means the transmission control module has detected an intermittent fault in the shift timing solenoid 'A' circuit or its operation. The module saw the solenoid's electrical signal or its expected response drop out or behave erratically from time to time, rather than fail permanently. Which solenoid is 'A' and how it is used depends on the vehicle and transmission.

Symptoms

• Gearshifts that are occasionally harsh, flared or delayed
• Intermittent limp mode or the gearbox stuck in one gear
• Transmission warning light or engine warning light coming on and off
• Erratic or unexpected shift timing, especially when warm
• Poor fuel economy or hesitation during gear changes

Possible causes

• Intermittent wiring or connector fault in the solenoid circuit (chafing, corrosion, loose pin, poor terminal tension)likely
• Faulty or sticking shift timing solenoid A with internal winding or mechanical intermittent faultlikely
• Low, degraded or contaminated transmission fluid causing solenoid sticking or erratic pressurepossible
• Blocked internal filter or debris in the valve body affecting solenoid operationpossible
• Poor power supply or earth to the transmission control module or solenoid, such as a weak battery or poor chargingpossible
• Fault inside the transmission control module or an outdated calibration known for this issue on certain modelsrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, including temperature, speed and load when the fault set
2. Check the transmission fluid level and condition against the manufacturer's procedure, looking for burnt smell, dark colour or debris
3. Check battery condition and charging voltage, as low voltage can cause intermittent solenoid faults
4. Inspect the transmission harness and connector for chafing, fluid ingress, corrosion, bent or pushed-back pins, and wiggle test it while monitoring live data
5. Measure the solenoid resistance and compare it with the specification for that model
6. Check the supply and earth to the solenoid and control module, and test the control circuit for intermittent open circuit or short
7. Monitor commanded versus actual gear and solenoid duty cycle in live data during a road test to catch the fault
8. Check for manufacturer service bulletins or calibration updates relevant to the vehicle and transmission

How to fix

• Repair or replace damaged wiring, and clean or replace corroded or loose connector terminals
• Replace the shift timing solenoid A if it fails electrical or operational tests
• Correct the fluid level, or change the fluid and filter using the specified type if it is degraded or contaminated
• Clean or overhaul the valve body, or repair the transmission if debris or wear is found
• Repair the battery, charging or earth faults that affect the control module supply
• Replace or repair the control module, or apply an updated calibration only if a known fault has been confirmed

Can I keep driving?

Nearby codes

• P0788GenericShift Timing Solenoid A High
• P078AGenericShift Timing Solenoid B