The OBD2 code P0778 refers to an issue with the electrical circuit of the Pressure Control Solenoid B in the transmission. This solenoid helps regulate the hydraulic pressure for shifting gears, and a fault in its electrical circuit can lead to noticeable transmission problems.
P0778 quick overview
| Meaning | P0778: Pressure Control Solenoid B Electrical |
| Is it serious? | Yes, it can lead to transmission shifting issues, affecting the vehicle’s drivability. |
| Possible causes | – Faulty or damaged wiring and connectors – Bad ground connection – Defective pressure control solenoid – Transmission control module (TCM) issues |
| How to diagnose? | Check wiring and connectors for visible damage or corrosion. Inspect the ground connection at the intake and transmission. Test the pressure control solenoid for proper resistance and operation. Examine the transmission control module (TCM) for signs of damage or corrosion. |
P0778 Meaning
For those not familiar with car mechanics, think of your transmission like a bicycle’s gears, shifting up or down as you ride. The Pressure Control Solenoid B is an electrically-operated valve that helps manage how these “gears” shift by controlling the oil pressure. If there’s an electrical problem with this solenoid, your car might shift gears too hard, too softly, or sometimes not at all. It’s like riding a bike where the gears won’t change properly, making it hard to ride smoothly or efficiently.
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How to fix P0778 – learn from mechanics
I looked into X service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.
Case #1: Subaru
Vehicle information 2001 Subaru Legacy GT 2.5L
Issue + repair history
- Transmission replaced
- Starts in 1st, shifts to 3rd, won’t shift manually
- Downshifts when stopped
- Oil temp light flashes from startup
- Only OBD2 code: P0778
Other OBD2 Codes + Test Results
- P0778
Mechanic suggestions
- Check three-wire ground at the intake, might be loose or bad
- Inspect wires at the left side intake manifold or harness to the transmission for damage
- Ground issues are common, ensure grounds are intact
Final fix
- Dropped pan and repaired three shorted wires due to pan pushing wires against valve body
Case #2: Volkswagen
Vehicle information 2002 Volkswagen Passat GLS 1.8L
Issue + repair history
- Shifts very hard
- Previous diagnosis suggested an electrical problem
- Code 18227: Pressure control valve 2 open or short to ground (P0778)
Other OBD2 Codes + Test Results
- P0778
- 18227: Pressure control valve 2 open or short to ground
Mechanic suggestions
- Double check grounds going to the computer
- Correctly identify Pressure Control Solenoid 2 and check for intermittent open circuit caused by coil winding break
- Perform an ohm meter test on the solenoid immersed in ATF heated to 100°C
- Consider replacing the valve body due to a history of failures
- Test the solenoid using a scan tool to command it on and off, and perform a voltage drop test from TCM to solenoid to check for thin or nearly broken wires
Final fix
- Replaced transmission computer due to water traces inside causing corrosion
Case #3: Mazda
Vehicle information 2003 Mazda 6 i 2.3L
Issue + repair history
- Intermittent hard shift
- Transmission light flashing
- OBD2 code: P0778
Other OBD2 Codes + Test Results
- P0778
Mechanic suggestions
- Perform a line pressure and stall test before considering solenoid replacement
- Check the light green and red wire at the transmission connector harness side for the solenoid
- Verify solenoid resistance between 2.6-3.2 ohms and check TCM resistance
- Confirm the issue is electrical, not mechanical, by ohming out the solenoid and checking terminal resistance to ground
Final fix
- Replaced defective 2-4 shift solenoid with part number FP03-21-1DO
How to diagnose P0778
I made you an overview of solving this DTC, based on all the advice that was suggested by certified mechanics to address this issue in previously mentioned service cases.
| Diagnostic Step | Brief Explanation |
|---|---|
| Check ground wires | Ensure the grounding connection is secure; loose or bad grounds can cause electrical issues. |
| Inspect wires and harness to the transmission | Look for damaged wires that could be causing short circuits. |
| Test pressure control solenoid and TCM wiring | Use an ohm meter and scan tool to check for proper solenoid operation and wiring integrity. |
| Replace defective parts | Depending on the findings, this may include the solenoid itself, the transmission computer, or repairing damaged wires. |
These cases highlight the importance of thorough diagnostics, including checking the wiring and ground connections, testing the solenoid and TCM, and ultimately addressing the specific cause of the P0778 code to restore proper transmission function.
Car Diagnostics Training
- Car Electrical basics
- OBD-II scanner basics
- How to use an OBD reader
- Introduction to a Bi-directional Scan tool
- OBD scanner buyer’s guide + discounts
I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko