C5 A6 Quattro 4.2, Transmission shifter will not shift into drive or lower gears.
 

I have a 2001 C5 A6 4.2l Quattro. The brake interlock works. The shifter moves from park to reverse and the car will move in reverse. The shifter moves from Reverse to N and the car sits still and idles. The shifter appears to move from N to Drive but does not quite go exactly all the way into Drive. When this happens, the car will not move forward and the lights on the dash do not highlight on the D position. Also because of this, when you shift to Tip mode it does not change the instrument cluster to indicate manual shift mode. All lights stay, none flash, and the D is not highlighted. The shifter will not shift from D into any lower forward gear position! This all occurred after I washed out the drain under the brake booster in the middle firewall. At first I thought I washed a stick or rock down onto the gear position switch and jammed the cable from full motion. After review, this is not the case. I removed the shifter covers and parts next. There appears to be nothing dropped down into the shift mechanism jamming the movement.

Could the gear position switch be failed internally, halting full range of motion to the lever?? Several hundred miles ago, I changed the filter and fluid. Could the filter have dropped, jamming the internal linkage for the shifter??

What would you check next. None of the options I can think of are very appealing. Will be a pain to remove the shift switch just to see if it frees stuff up. Will be a bigger pain to drain new fluid just to look inside (might be VERY careful and reuse that fluid)!

Thoughts?

Thanks!

Side note, I have ability to scan the car but have not yet (I know, I know)! Will check that next, though I feel there will be nothing there. I think the car is doing what it thinks it should be doing. It feels very mechanical in nature to me.