|Code||Fault Location||Probable Cause|
|P0748|| Transmission fluid pressure (TFP) solenoid -electrical |
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|Wiring, TFP solenoid, ECM/PCM!TCM|
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What Does Code P0748 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.
What are the common causes of code P0748?
Some common causes of code P0748 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
- Degraded transmission fluid
- Defective pressure control solenoid
- Defective transmission valve body, or clogged fluid passages in the valve body
- Mechanical failure of the transmission pressure pump
- Failed or failing PCM/TCM. Note that control module failure is rare, and the fault must be sought elsewhere before any control module is replaced
What are the symptoms of code P0748?
Some common symptoms of code P0748 could include the following-
- Stored trouble code and possibly an illuminated warning light
- Other transmission related codes could be present
- Varying degrees of transmission slippage may be present
- Transmission may overheat
- Transmission may be stuck in one gear, or may not engage some gears
- Misfire-like symptoms may be present at some engine speeds
- Transmission may enter a fail-safe or limp mode
- Fuel consumption may increase considerably
How do you troubleshoot code P0748?
Record all fault codes present, as well as all available freeze frame data. This information could be of use should an intermittent fault be diagnosed later on.
Determine the condition and level of the transmission fluid. If the fluid is level is low, adjust the level, but be sure to locate and repair all fluid leaks before returning the vehicle to service.
If the fluid is dark in color, has a “burnt” odor, or has a thick, tarry consistency, the fluid is degraded and it must be replaced along with the fluid filter and oil pan gasket. Be sure to follow the directions in the manual exactly to prevent damage to the transmission.
NOTE: If the fluid cannot be inspected or replaced because the transmission has a “life-time” fill, the wiser option is to refer the vehicle to the dealer or other competent repair facility for professional diagnosis and repair.
Refer to the manual to locate the transmission wiring harness, and perform a thorough visual inspection of the entire harness. Look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Replace wiring or make repairs as required if damaged wiring is found.
If the code persists but the wiring is free of damage, prepare to perform resistance, continuity, and ground integrity checks on all associated wiring, but be sure to determine the function and color coding of all relevant wiring before testing any circuits to prevent causing short circuits. Also be sure to disconnect all wiring from all related control modules to prevent damaging these control modules during resistance tests.
Bear in mind that while testing procedures and electrical values differ between applications, in most cases there should be zero resistance on circuits and across connectors. If the resistance on any circuit exceeds zero ohms (or the value specified in the manual), the circuit is shorted; if there is no continuity on any circuit that circuit is open.
If deviations from stated values are found, make repairs or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.
NOTE: If defects in the transmission’s internal wiring are identified, bear in mind that the entire internal harness must be replaced to possible avoid issues with short circuits and excessive resistances later on.
If steps 1 through 5 did not resolve the problem, the better option is to refer the vehicle for professional diagnosis and repair by the dealer or other competent repair facility, since additional diagnostic steps usually involve removal and partial or even complete disassembly of the transmission.