Fix P0758: Mechanic’s solutions that WORKED

The code P0758 Shift Solenoid B Electrical Code signifies a fault in the electrical circuit of Shift Solenoid B. Shift solenoids play a crucial role in automatic transmission operation, controlling the flow of transmission fluid to engage specific gears. In this case, Solenoid B is not functioning correctly electrically.

P0758 quick overview

Meaning	P0758: Shift Solenoid B Electrical Code
Is it serious?	Yes, this code should be addressed promptly.
Possible causes	– Faulty Shift Solenoid B. – Wiring or harness issues in the solenoid circuit. – Poor electrical connections or corroded terminals. – Problems with the Transmission Control Module (TCM). – Internal transmission issues, although less common.
How to diagnose?	Check for Other Fault Codes: Start by checking for any additional fault codes present in the vehicle’s computer system, as they may provide additional clues. Visual Inspection: Visually inspect the wiring harness and connectors for signs of damage, corrosion, or loose connections. Test Solenoid B: Measure the resistance of Shift Solenoid B to determine if it falls within the specified range. If it’s out of range, consider replacing the solenoid. Check Wiring: Perform a thorough inspection of the wiring associated with Shift Solenoid B, checking for continuity and shorts. Inspect Transmission Fluid: Ensure the transmission fluid is at the correct level and in good condition. Contaminated or low fluid can affect solenoid operation. Examine TCM: If all other tests are inconclusive, consider inspecting the Transmission Control Module (TCM) for faults or errors. Road Test: If no issues are found during the previous steps, conduct a road test to observe transmission behavior and verify if the problem persists.

How to fix P0758 – learn from mechanics

I looked into P0758 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: Chevrolet

Vehicle Information:
1996 Chevrolet Suburban 5.7L

Issue + Repair History:
The vehicle had the Service Engine Soon (SES) light on and defaulted to third gear. The following fault codes were retrieved:

• P0758 (B Solenoid Fault)
• P1860
• P1864
• P1886
• P0758
• P0753

The 2-3 shift solenoid was tested and found to be faulty, so it was replaced. Additionally, a damaged connector was discovered in the transmission pan, leading to a replacement of the wire harness. After these repairs, the SES light remained on with no pending codes, but memory codes (P1860, P1864, P1886, P0758, and P0753) persisted.

Other OBD2 Codes + Test Results:
The manual cycle of the solenoid was tested and was found to be working correctly.

Suggestions from the Community:
One suggestion was to check the ignition switch (electrical part) to ensure it was functioning correctly.

Final Fix:
The final fix involved rewiring the Vehicle Control Module (VCM) with a service kit (kit #12167310) to add a ground to the circuit boards. This addressed the issue caused by bad grounds on the internal boards of the VCM, resolving the problem.

Case #2: GMC

Vehicle Information:
1995 GMC Jimmy SL 4.3L

Issue + Repair History:
The problem occurred intermittently, especially when the vehicle was wet. The Vehicle Control Module (VCM) set code P0758 (Solenoid B Error) and went into limp mode, not grounding solenoids. Sometimes, the issue would reset with a restart.

Other OBD2 Codes + Test Results:
The only mentioned code was P0758 (B Solenoid Error).

Suggestions from the Community:
The technician suspected a harness problem, especially because the issue seemed to occur when the vehicle was wet.

Final Fix:
The issue was resolved by spraying insul-spray sealer on the connector. This fix effectively addressed the problem caused by moisture affecting the connector.

Case #3: Pontiac

Vehicle Information:
1997 Pontiac Firebird Formula 5.7L

Issue + Repair History:
The Check Engine Light (SES) came on, and the vehicle experienced a loss of power. The scan tool revealed code P0758, indicating a 2-3 shift solenoid electrical issue. The problem seemed to worsen as the transmission got hotter.

Other OBD2 Codes + Test Results:
The only mentioned code was P0758 (2-3 Shift Solenoid Electrical).

Suggestions from the Community:
Several community members suggested replacing the 2-3 shift solenoid. The technician followed this advice and cleared the codes, but the same code (P0758) reappeared. The technician then checked the wire harness, which appeared to be in good condition. After clearing the codes again, the transmission shifted fine, and the SES light did not come back on. The issue might have been due to a loose connection or a possible ECM malfunction.

Final Fix:
The final fix was not explicitly mentioned, but after checking the wire harness and clearing the codes, the transmission shifted correctly, and the Check Engine Light did not return. The exact cause of the issue, whether it was a loose connection or an intermittent ECM problem, was not specified.

Case #4: Mazda

Vehicle Information:
2011 Mazda 6 i 2.5L

Issue + Repair History:
The technician encountered a P0758 trouble code, suggesting a problem with Shift Solenoid B in the vehicle’s transmission. The dealer mentioned they had never sold a solenoid or valve body for this particular car type. The technician inquired about the voltage they should see at pin C from the TCM (Transmission Control Module), suspecting it should be 12 volts.

Other OBD2 Codes + Test Results:
The only mentioned code was P0758 (Shift Solenoid B Electrical). The technician also performed some tests:

• Checked the solenoids’ resistance, and they were within specifications.
• Applied voltage to the solenoids and could hear solenoids A, C, and G clicking but did not hear anything from solenoids B and F.
• Tested for voltage at the connector on terminal C and found only 5.0 volts.

Suggestions from the Community:
The community provided some suggestions for diagnosing the issue. One member recommended checking for shorts or grounds in the wires and looking for a good diagnostic chart for the P0758 code. Another member mentioned that the solenoids should all operate from 12 volts.

Final Fix:
The final fix involved the technician installing an oscilloscope on the terminals of the TCM and comparing the input/output wave from the TCM. They observed a problem with the module. Solenoids A, B, and C should have the same wave pattern, but they found 5 volts in park and 0 volts in gear, which indicated an issue. The technician installed a new TCM after checking for shorts or grounds in the wires, and this resolved the problem. They also discovered corrosion inside the old module, possibly due to water exposure, which might have caused the issue.

Case #5: Lexus

Vehicle Information:
1999 Lexus RX300 3.0L

Issue + Repair History:
The customer worked on the vehicle, replacing both oil control valves, the MAF sensor, timing belt, front A/F ratio sensors, and valve cover gaskets. The problem they encountered was poor idle when the engine is cold.

Other OBD2 Codes + Test Results:
The technician noticed that the engine runs smoother when the alternator is unplugged. When scoping the CKP (Crankshaft Position) and IGF (Ignition Feedback) signals, both signals are pulled down when the alternator is plugged in. CKP and IGF signals are affected when the battery voltage is increased, either by the alternator or a jump box being turned on. The technician also observed misfires at cylinders #1, #3, and #5 on the misfire monitor when test driving with the alternator disconnected.

Suggestions from the Community:
The community provided suggestions for checking grounds on the back of the intake plenum, as issues with these grounds are common. They also suggested inspecting fuel injectors for cleanliness, as dirty injectors can affect idle quality, especially when the engine is cold.

Final Fix:
The technician found the root cause of the issue to be a loose ground. They discovered an additional ground eyelet tucked under some brake lines on the bulkhead at the back of the plenum. After re-fitting this ground, the problem was resolved.

Computer Codes & Descriptions:

• P0758 (Shift Solenoid Bank 2 circuit fault)

It’s important to note that there were no trans codes mentioned in the final fix, which indicates that the issue was related to the ground and not the transmission.