Overdrive not working

On some makes of vehicles the overdrive light flashes because the computer has logged a fault code,sometimes because it is overtemp eg too hot (maybe trans fluid issue might require servicing,or fluid level is low. Try checking fluid level and condition if possible. Good luck there.

Hope it is a solenoid issue or your pulling the trans out
Try a trans shop
The shift solenoid should be accessible removing the tran pan
It is 14 years old so don't get you hopes up for a simple fix

Drain fluid into a clean plastic bowl, check for brass cuttings or metal shavings in sunlight. If present get complete trans overhaul.

was the computer for that exact year and model ? computers have to be matched to that same model to run the same transmission as in the vehicle example 1999 ford crown vic trans.put in a 2000 crown vic even though the trans. is the same the ccomputer reads trans. is built for 1999 cause of the program in the comp.

sounds like over drive switch or transmision box

If you mean engine wont stay running in drive might be looking at torque converter, if you mean wont go into over drive even with your over drive switch on your over drive clutches and or shift solenoids are bad trans needs rebuilt or replaced

is this a three speed trans? or 4spd w/overdrive? either way i believe you have a pressure problem whether a torque converter or a trans pump. I was wondering how you know all gears worked if it will only do 30 mph as the 4th or overdrive usually wont engage at that speed. but here say,

get it to your local trans shop for a diagnostic, sounds like rebuild time.

no. its part of the valve body inside of the transmission. you would need to drain and remove the transmission oil pan and then remove the valve body. Best left to a transmission shop or reputable repair center.

Try to adjust the linkage cable on the transmission; if that don't work change the transmission fluid and filter.