P0788 OBD-II Trouble Code: Shift/Timing Solenoid High

P0788 Code Definition

The P0788 code is a generic powertrain code related to the transmission. When this code is stored, it means that the engine control module (ECM) has found a problem with the shift/timing solenoid, specifically that the voltage to the timing valve is registering higher than allowed preset parameters.

Codes related to the P0788 include:

• P0785: Shift/Timing Solenoid
• P0786: Shift/Timing Solenoid Range/Performance
• P0787: Shift/Timing Solenoid Low
• P0789: Shift/Timing Solenoid Intermittent

What the P0788 Code Means

When a code P0788 is stored, the ECM has detected a high voltage from the timing solenoid valve transmission control module (TCM) terminal, even though the TCM is commanding the timing solenoid valve to turn off. The ECM may be registering that the vehicle is in a different gear than it actually is.

Shift solenoids are used by an automatic transmission to manage the transfer of fluid between hydraulic circuits in order to make a change in the transmission gear ratio.

What Causes the P0788 Code?

Several problems may cause a P0788 code including:

• Low or dirty transmission fluid
• Damaged or defective shift solenoid
• Open or shorted shifting solenoid valve harness
• Poor electrical connection to the shifting solenoid valve circuit
• Blockages in the transmission fluid passages
• Damaged or defective transmission control module (TCM)
• Internal mechanical failures in the transmission
• Damaged or defective ECM (rare)

What are the Symptoms of the P0788 Code?

Symptoms of a P0788 code may include:

• Warning or check engine light may illuminate
• The vehicle may not exhibit any signs at all, other than a stored P0788 code
• Transmission may not shift into, or from, a specific gear
• Transmission may shift roughly from one gear to another
• May experience transmission slippage
• Transmission may overheat
• Decrease in fuel efficiency
• Vehicle may enter into “limp mode”

How Does a Mechanic Diagnose the P0788 Code?

A mechanic may use several methods to diagnose a P0788 trouble code, including:

• Utilize an OBD-II scanner to check for a stored P0788 trouble code
• Check transmission fluid level and clarity
• Check transmission fluid pressure
• Visually inspect for any damaged or loose wiring, harnesses, or connectors
• Check for any transmission fluid leaks
• Use a manual pressure gauge to check transmission pump pressure
• Check for clogged hydraulic passages
• Determine if pump, shift solenoid, or electronic pressure sensor is damaged or defective
• Check reference voltage and ground signals on the affected shift solenoid
• Test all related circuits for resistance

Common Mistakes when Diagnosing the P0788 Code

It’s possible to diagnose a P0788 code as a faulty shift solenoid issue, when the problem is an issue related to the shift solenoid circuit. It is recommended to check and test all electrical wiring and circuitry carefully before proceeding with repairs.

How Serious is the P0788 Code?

Because a vehicle’s safe driving operation may be impacted due to faults that store a P0788 code, this code is considered a potentially serious code. When this code appears, it is recommended to immediately take your vehicle to your local service center or mechanic for repair and diagnosis.

What Repairs can Fix the P0788 Code?

Several repairs can fix a P0788 trouble code and include:

• Flush and/or fill the transmission fluid to manufacturer specific levels
• Visually inspect transmission fluid, and smell for any burning smells
• Repair any loose or damaged electrical wires, connectors, or harnesses
• Repair or replace damaged circuits and connectors
• Repair or replace damaged or defective pump
• Repair or replace damaged or defective shift solenoid or solenoid pack
• Repair or replace electronic pressure regulator
• Replace damaged or defective ECM (rare)
• Clear all codes, test-drive the vehicle, and rescan to see if any codes reappear
• Check transmission pan for any contamination or debris, which if found, will require a total transmission rebuild, and a new torque converter

Additional Comments for Consideration Regarding the P0788 Code

Typically, additional shift solenoid codes will be stored along with the P0788 code. It is recommended to always diagnose and repair trouble codes in the order in which they appear.

Need Help with a P0788 Code?

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