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Error Code P0748 is defined as Pressure Control Solenoid A Electrical. This is a generic trouble code, meaning it applies to all vehicles with the OBD-II system, especially those made since 1996 up to present and equipped with automatic transmission. This includes vehicles from, but not limited to, Acura, Allison/Duramax, Chevrolet, Dodge, Ford, Honda, Jeep, Lincoln, Mercury, Jaguar, Toyota, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.
When Error Code P0745 I set, that means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has determined a problem with the Transmission Pressure Control Solenoid “A”. Most vehicles equipped with automatic transmission comes with at least three solenoids (solenoid A, B, and C). Trouble codes related to “A” solenoid include:
These codes are based on a specific malfunction that alerts the PCM and activates the Check Engine light.
The job of the Transmission Pressure Control Solenoid is to control the pressure of the fluid for its right operation of the automatic transmission. The PCM gets an electronic signal based on the pressure with the solenoids. Automatic transmissions are controlled by bands and clutches that changes gears using fluid pressure in the right place, at the right time. Based on the signals coming from associated devices that monitor the vehicle’s speed, the PCM controls the pressure solenoids to direct fluids at the appropriate pressure, to various hydraulic circuits that change transmission gear ratio at the right time.
Error Code P0748 is set when the Pressure Control Solenoid “A” is experiencing an electrical malfunction.
In severe cases, the vehicle cannot be driven when this code appears. Other common symptoms include:
- Vehicle in limp mode
- Overheated transmission
- Transmission slipping when shifting
- Transmission catches in gear
- Increase in fuel consumption
- Misfire like symptoms
Defective valve body or pressure control solenoid are the two most common causes of this code. In many cases, the vehicle may also be suffering from defective wiring. Other possible causes include:
- Dirty or contaminated transmission
- Dirty or clogged transmission filter
- Restricted hydraulic passages
- Defective transmission pump
- Corroded or damaged connector
- Faulty PCM
How to Check
Before starting with any troubleshooting process, it’s important to check the with vehicle’s TSB (Technical Service Bulletins), specific to its year, model, and transmission. In some cases, this can help save a lot of time, as it points you in the right direction. Also, it pays to check the vehicle’s records to verify the last time the fluid and filter was changed.
Inspect Fluid and Wiring
First, check the transmission fluid’s condition and level. Before this, you can consult with the vehicle’s records to verify the last time the fluid and filter was changed.
For the wirings, it’s obvious. Make sure the connectors and connections are secured, and there are no signs of damages and corrosions. This includes inspecting all wirings and connectors to the transmission pressure control solenoids, transmission pump, and PCM. Depending on the configuration, the transmission pump can be electrically or mechanically driven.
These steps may vary from one vehicle to another, and requires advanced equipment to perform, such as digital multi-meter and specific technical references for the vehicle. You will need the specific troubleshooting guidelines for your vehicle prior to proceeding with this kind of diagnosis.
Also, remember that voltage conditions may vary based on the model of the vehicle. Fluid pressure requirements can also vary based on the design and configuration of the transmission.
Unless it was specified in the technical data, the normal resistance reading for connections and wirings is 0 ohms. Continuity check should be performed with the power removed from the circuit to avoid shorting the circuit and doing more damage. Resistance or no continuity is a sign of faulty wiring, which may be open or shorted, and must be repaired, if not replaced.
How to Fix
Depending on the diagnosis, common repairs for this problem include:
- Changing the fluid and fluid filter
- Repair or replacement of defective transmission pump
- Replacement of defective pressure control solenoid
- Repair or replacement of defective transmission valve body
- Cleaning the fluid passage through fluid flush
- Cleaning connectors off of corrosions
- Repair or replacement of wirings
- Flashing or replacement of PCM
This code is considered a serious problem in most cases, as it can keep the vehicle from being driven.
The most common misdiagnosis for this code includes engine misfire problem, internal transmission problem, transmission pump problem, and driveline problem. Thus, a thorough diagnosis is important, especially in the valve body or solenoid.