I have a 2006 Ford st Mk2 225. Driving down the road and the dash lit up like a Christmas tree and would not let me select 5th gear. Its like 5 doesn't exist as the stick won't even try to go in to gear. Upon returning to home and plugging it in to a code reader is come back with p0500. My understanding is that this is the gearbox speed sensor. I have read today that there is no such thing on the St and that the abs sensors could cause the gears to become like this as well as the abs and traction light on the dash. I really am a bit stuck as I would be a coincidence if 2 parts have failed at the same time. Any help would be appreciated. 

I'd be very surprised if you've damaged the gearbox. The 6 speed on the ST and RS are solid boxes.

I'd check the solenoid connected to the shifter. It prevents you from selecting reverse instead of 6th gear but also stops you using 5th and 6th gear when in limp mode. If you remove fuse 100, it disables the solenoid (along with auto level headlights and a couple other control modules) and should allow you to select whatever gear (as long as it is the solenoid at fault lol).

5 minutes ago, Luke4efc said:

...also stops you using 5th and 6th gear when in limp mode.

Never knew that, how interesting!  

2 hours ago, TomsFocus said:

Never knew that, how interesting!  

Reminds me of the movie "Short Circuit" Johnny five is Alive 😅