Hello,

* The car is S80 2.0T (2012) with DCT450 transmission (MPS6)

* A (Transmission Performance Low) message is shown.[only 2 & 3 gear are working with no reverse]

* when scanning the car error code (P172C00) is shown as the below image [Shift Fork A/B Direction Control Valve Stuck ON]

* couldn't find details on that error code; code anyone help with some details about the error code and how to solve the problem please.

Many thanks in advance.

 

just reading the code, it sounds like the solenoid that actuates the shift fork is unable to move the the fork, the solenoid/control valve isn't getting the correct signal or the valve itself is bad. A dual clutch manual (like VWs DSG) is really a manual transmission with computer controlled switches to control the hydraulics that do the job of the shifter and clutch pedal. Interestingly its like two transmissions spliced together (ie odd vs even speeds) so one stuck shift fork will take out the gears on that particular shaft. So the possible causes would be wiring/connectors from the TCM to the transmission, mechanical issue within the transmission or the valve itself. Good news is you can find vids on youtube and repair manuals (for purchase) online. Bad news is a replacement valve body is gonna be expensive (like $1500+). I'd probably start by checking out wiring/connectors, reseat the TCM, check for anything going on in the shifter console (like spilled coffee?) cleaning contacts along the way.