The P0748 code is an issue with the electrical circuit of the Pressure Control Solenoid ‘A’ in the transmission. This solenoid is crucial for controlling the hydraulic pressure used to shift gears. When the vehicle’s computer detects an electrical problem with this solenoid, such as incorrect voltage or resistance, it triggers the P0748 code. This can lead to transmission performance issues, including improper shifting or the vehicle starting in a higher gear and limiting the available gear range.
P0748 quick overview
| Meaning | P0748 – Pressure Control Solenoid ‘A’ Electrical |
| Is it serious? | Yes, this issue can affect the drivability and performance of your vehicle. |
| Possible causes | – Faulty pressure control solenoid – Wiring or connector problems in the solenoid circuit – Low transmission fluid level or fluid quality issues – Malfunctioning Transmission Control Module (TCM) or Powertrain Control Module (PCM) |
| How to diagnose? | Inspect the pressure control solenoid for damage or failure. Check the wiring and connectors for shorts, opens, or poor connections. Verify transmission fluid level and quality, replacing or flushing if necessary. Test the electrical circuit for proper voltage and ground at the solenoid. Consult vehicle-specific wiring diagrams for accurate diagnosis and repair. |
P0748 Meaning
When your car throws a P0748 code, it’s signaling that there’s an electrical hiccup with one of the tiny valves (solenoids) that help manage how your transmission shifts gears by controlling oil pressure. If this solenoid isn’t working right because of a wiring issue, a bad connection, or even if it’s just the wrong type for your car, you might notice your ride doesn’t shift like it should.
It could get stuck in a high gear right from the start or refuse to shift up through the gears as you speed up. Fixing it might involve checking the solenoid itself, making sure there’s enough clean transmission fluid, and ensuring all the electrical connections are snug and not damaged.
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How to fix P0748 – learn from mechanics
I looked into P0748 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: Dodge
Vehicle Information 1996 Dodge Ram 1500 5.9L
Issue + Repair History
- Truck starts out in second gear and will not shift.
- Shifting to manual low allows shifting up, but never goes to fourth gear.
- Previous repairs:
- Replaced governor pressure sensor and solenoid.
- Fluid and filter replacement.
Other OBD2 Codes + Test Results
- DTC: P0748
Mechanic Suggestions
- Install a pressure gauge on the governor tap on the top of the transmission case to monitor pressure while driving.
- Check the output speed sensor and ensure the fluid level is correct, as low fluid levels can trigger limp mode.
- Perform a complete diagnosis of the vehicle to identify any other underlying issues.
- Consider the possibility of a malfunctioning PCM, as it’s a known issue for this model year.
- Check for a blocked cooler that may be preventing lubrication to the overdrive planetary.
Final Fix
- Remanufactured transmission fixed the vehicle’s issue.
Case #2: Dodge
Vehicle Information 1997 Chevrolet K2500 Cheyenne 6.5L
Issue + Repair History
- Continuously experiencing trouble code P0748 despite previous repair attempts.
- Previous repairs:
- Replaced Pressure Control Solenoid (PCS).
- Replaced internal transmission harness.
- Replaced harness from PCM to transmission.
- Replaced PCM.
- Installed new PCS 2″ away from PCM.
- Commanded PCS on with scanner but actual reading always showed 0 amps.
Other OBD2 Codes + Test Results
- DTC: P0748
Mechanic Suggestions
- Check for power-related issues such as fuse connection problems or ignition switch issues.
- Perform a volt drop test on the positive and negative sides of the harness, ensuring to test with the circuit loaded.
- Test resistance of the PC Solenoid Valve (PC Sol. Valve) and the Automatic Transmission Wiring Harness Assembly.
- Consider the possibility of a faulty PCM.
- Verify the correct PCS is being used and ensure GM parts are used.
- Inspect circuitry for any shorts, opens, or grounds.
- Consider replacing the PCM if necessary.
Final Fix
- Discovered an inaccurate wiring diagram from Mitchell, received a corrected diagram from ATRA.
- Identified an open in the wire for the PCS low side.
- Replaced the open wire, resolving the issue.
Case #3: Subaru
Vehicle Information 1999 Subaru Legacy Limited 30th Anniversary 2.5L
Issue + Repair History
- Vehicle initially came in with major bearing and differential noises.
- Transmission seemed to shift okay, and no service lights were illuminated.
- Offered customer option of rebuild or replacement; customer chose replacement with used transmission.
- After replacement, transmission temperature light flashed after startup, and trouble code P0748 (duty solenoid A open or shorted) appeared.
- Replaced transmission twice due to persistent issues with shifting failure and recurrence of code P0748.
- Tested solenoid A and dropping resistor, which showed normal readings.
- Transmission remained stuck in 2nd gear, resembling limp mode.
- Considered various possibilities including TCM, wiring, and transmission pump/torque converter.
Other OBD2 Codes + Test Results
- DTC: P0748
Mechanic Suggestions
- Inspect transmission pan for possible contact causing damage to solenoids or wiring harness.
- Check if software needs to be recoded for the new transmission.
- Consider the possibility of damaged solenoids due to jacking on the transmission pan.
- Inspect original transmission for signs of obvious problems.
- Test voltage signals for any abnormalities.
- Ensure proper connections of original trans wiring harness and neutral safety switch.
- Check for bent or damaged pins in connectors and ensure all pins are securely connected.
- Inspect driveshaft, rear differential, and tire tread depth for potential sources of noise.
- Check PRND switch at the transmission, connector, and wiring.
- Verify circuit resistance from TCM connector and check voltages at TCM connector pins.
- Double-check harness connectors at the bellhousing for proper connection and condition of terminals.
- Ensure all grounds are good.
Final Fix
- Completely rewired transmission wiring from trans connector on bell housing to bypass the connector altogether.
- The issue was resolved, and the vehicle no longer exhibited the flashing trans temp light or set code P0748 after the rewiring.
How to diagnose P0748
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 | Explanation |
|---|---|
| Inspect the solenoid and wiring | Look for visible damage or disconnections that could cause electrical faults. |
| Check transmission fluid | Ensure the fluid is at the correct level and in good condition to prevent pressure issues. |
| Test electrical connections | Use a multimeter to check for proper voltage and ground in the solenoid circuit. |
| Consult wiring diagrams | Accurate diagrams are crucial for tracing the solenoid circuit and identifying potential faults. |
| Rewire if necessary | In complex cases, bypassing damaged connectors or wiring may resolve the issue. |
In these cases, solutions ranged from replacing the transmission, and securing loose connections, to rewiring the solenoid circuit. Each fix underscores the importance of thorough diagnostics to accurately pinpoint and address the root cause of the P0748 code, ensuring the transmission functions correctly and extends the vehicle’s service life.
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