it is to select "overdrive" gear in transmission. mainly for highway driving. if you have both "D" and "D" with circle around it, use "D" for most driving needs.
Sounds like you have a bad Vehicle Speed Sensor.
Linkage is broken or loose - or trans pump shaft is broken ( big job)
That is a "Check transmission light"
Overdrive
Solenoid in trans
There are 2 excellent write-ups on the procedures for replacing the shifter cable, one for floor shift and column shift each, at the Taurus Car Club site. See Related links below for more information.
Replace your transmission pump or get a new transmission.
The O/D OFF warning light in the bottom left of your tachometer on your 1999 Mercury Sable means that the ( OVERDRIVE IS OFF ) * that's the " top gear " of your automatic transmission
you need a new transmission
When the transaxle light is blinking on a 2001 Mercury Sable, it means there are some transmission issues that need to be addressed immediately. This condition may lead to locking of the transmission on second gear.
There are a couple of reasons why a 1993 Mercury Villager is hard to shift. One reason could be a gear shift cable. Another reason could be low transmission fluid.
The one on the tachometer that is surrounded by a gear is the check transaxle warning light