1998 Mercury Sable

Transmission problem
1998 Mercury Sable 6 cyl Front Wheel Drive Automatic 160000 miles

Hello. My 1998 Sable will not shift into overdrive. It shifts into reverse and 1st-3rd fine, but once I get it up to around 50-60 it will not shift into overdrive. I've turned the overdrive off and on to no avail. The transmission fluid is a bit dirty, but none of the other gears have been affected. Running diagnostics showed that the EGR Valve and O2 sensor are bad. I've also been told a valve or sensor in the tranmission could be not working. Is this is a quick fix or a deeper internal transmission problem? Thank you!
July 7, 2009.

Depends what tranny you have in it
some require a complete tear down
do a code check
EGR code check DPFE sensor
and clean egr pass ways

Jul 12, 2009.