Liftmaster 1/2HP Opens then closes - RPM Sensor has already been replaced

Hi All,
I was recently adjusting the range on one of my garage door openers after replacing some damaged links and tensioning the chain.

During this adjustment, the DOWN setpoint screw dropped from the adjustment hole and into the chassis of the opener. 

I opened the back of the opener to clip the adustment screw assembly back into place. This required me to disconnect the RPM sensor header and move the up PCB out of the way to allow access.

I clipped the range screw assembly back into place and noted that the sliding contact was close to the down position limit contact (but not making contact) which looked about right given the slighly open position of the door so I replaced all the components to test.

On testing, the door moved about 2" then stopped and reversed. I have operated the door while viewing the limit contacts, and the moving contact never makes contact with the DOWN limit contact before the door reverses.

A little digging online indicated that this behaviour is usually due to a faulty RPM sensor, so I purchased a new one fron my local authorised parts supplier and installed it to no effect. The door still moves about 2" then returns, never making contact with the limit switches.

I did notice that when I installed the new RPM sensor I had inadvertantly re-connected the original sensor to the header one pin off (as the header is not keyed and the area I was working in is dark and very awkward to see in).

I wanted to put it out to the group if this misconnection has likely damaged the mainboard, and if there are steps I can take to fix the board (Im no stranger to electronics) I would just need to know where to start.

Otherwise outside of this, if there are any further troubleshooting steps to take that I may have overlooked.

TIA,
Jon.

Anybody any insight? Do I have to replace the whole thing? Not sure I can get just a new controller PCB where I live. Would prefer to fix!