What does it mean?
SPECIAL NOTES: Non-professional mechanics should note that if code P0746 occurs on an application with a “filled-for-life” transmission, the wiser option is to refer the vehicle to the dealer, or other competent repair shop for professional diagnosis and repair. “Filled-for-life” transmissions are not fitted with dipsticks with which to measure the fluid level, and determining the actual level of the fluid in these transmissions is all but impossible without equipment that only the dealers possess.
Thus, removing the oil pan on these transmissions to check the condition of the fluid, or to check for the presence of large wear particles (evidence of mechanical failure(s)) is NOT recommended, since it is nearly impossible to refill these transmissions with the correct amount of transmission fluid. Bear in mind that over filling a transmission is as bad as under filling it and either condition can cause catastrophic transmission failure. END OF SPECIAL NOTES.
OBD II fault code P0746 is a generic code that is defined as “Transmission fluid pressure (TFP) solenoid – Performance or Stuck Off”, and is set when the PCM (Powertrain Control Module) detects a difference between the desired transmission fluid pressure, and the actual transmission fluid pressure at any given moment. While most causes of this code involve malfunctions in the control circuit of the pressure control solenoid, be aware that there are many possible non-electrical causes as well.
NOTE: In this definition, the words “stuck off”, refers to a condition in which the solenoid that controls the transmission fluid pressure is stuck in the “OFF”, or closed position. In this condition, the pressure of the transmission fluid cannot be controlled, directed, or regulated at all. While the setting parameters for this code vary greatly between manufacturers, it should be noted that all automatic transmissions depend on pressurized transmission fluid to function properly.
The pressure is delivered /developed by a pressure pump that is driven by the engine via the torque converter, and effective control of the pressurized fluid is essential for efficient gear selection and torque converter lock-up clutch operation.
In terms of operating principles, pressurized transmission fluid needs to be circulated through the transmission casing to provide proper lubrication to moving parts, as well as to provide pressure to move, and maintain pressure on clutch packs and planetary gear sets to effect gear changes, and to maintain a particular gear ratio selection.
Thus, to ensure that all parts of the transmission is supplied with both an adequate flow of transmission fluid, and transmission fluid that is pressurized sufficiently to act effectively on moving parts, the fluid is passed through a dedicated pressure control solenoid.
Depending on operating conditions such as throttle opening, engine speed, transmission turbine speed, the currently selected gear, and the vehicle’s road speed, the PCM calculates the pressure required for the transmission to react appropriately to changing conditions.
For instance, based on current operating conditions such as hard acceleration, the PCM might instruct the TCM (Transmission Control Module) to initiate a measure of torque converter clutch slippage (to protect transmission components), before selecting a lower gear as an appropriate response to a large, and sudden throttle input.
To perform these actions effectively, the transmission fluid needs to be at a certain pressure, which the PCM will provide by adjusting the flow rate through the transmission fluid pressure-control solenoid by manipulating the solenoid’s opening. In a fully functional system, these actions are virtually seamless and the driver is not aware of the process.
However, in a system that can control neither the flow rate, nor the pressure of the transmission fluid, the transmission cannot react appropriately to inputs such as the large throttle input in our example above. When th