What does it mean?
SPECIAL NOTES: Non-professional mechanics should take note that DIY diagnostic and repair steps for code P0748 – “Transmission fluid pressure (TFP) solenoid –electrical” are limited to a few basic checks and tests that may or may not resolve the problem.
Also take note that the information provided here is intended for general informational purposes only, and should therefore NOT be used in any diagnostic/repair procedure for this code without making proper reference to the manual for the affected application.
If the few generic steps in this guide do not resolve the problem, refer the vehicle to the dealer or other competent repair facility for professional diagnosis and repair, and especially so in the case of transmissions that have “life-time” fluid fills. Transmissions of this type can generally only be drained and refilled with specialized equipment. END OF SPECIAL NOTES.
OBD II fault code P0748 is a generic code that is defined as “Transmission fluid pressure (TFP) solenoid –electrical”, and is set when the PCM (Powertrain Control Module) detects a failure or malfunction in either the electrical control circuit of the transmission pressure control solenoid, or a failure or malfunction of the pressure control solenoid itself.
NOTE: Take note that some, if not most modern applications/transmissions have more than one pressure control solenoid, and codes that relate to the different pressure control solenoids are invariably qualified with the letters “A”, “B”, and “C”. Note though that not all manufacturers label the solenoids (or label them in the same way) so if code P0748 is present, it more often than not refers to pressure control solenoid “A”.
However, always refer to the manual for the application to determine exactly which solenoid code P0748 refers to on that application. The sole purpose of a transmission pressure control solenoid is to control/regulate the pressure of the transmission fluid to ensure efficient operation of the transmission in all gears.
In practice though, when a gear shift is initiated the pressure control solenoid diverts pressurized fluid from one hydraulic circuit to another to maintain the required pressure in that circuit to keep planetary gears and clutches engaged. Insufficient pressure can cause transmission slippage, while excessive pressure can cause overheating of the transmission that can damage some transmission components.
To keep the transmission fluid pressure within design limits, the PCM continually monitors the actual fluid pressure at various points in the transmission, and compares this pressure to the desired pressure. When the PCM detects an electrical failure or malfunction in a pressure control solenoid control circuit (or in the solenoid itself) that causes the actual pressure to differ from the desired pressure by a preset amount, it will set code P0748, and may also illuminate a warning light.
The image below shows a typical transmission pressure control solenoid, such as might be found on almost any modern transmission.