[jdmccright]

A few more basic questions I should have asked:

1) Model year?
2) Engine type & size
3) Your location (country is enough)?

Did you have the engine light checked out three years ago or just recently? I ask because once it is on, it won't be able to let you know if more problems are detected (with the exception of a serious problem then it would flash). However, when it was checked, the mech should have reset the light by clearing all the trouble codes to see if it returns. First thing to do would be to go and ask what the specific trouble codes that have been stored in the vehicle's computer (or ECU) and clear them from the memory and post those here. Then if the light comes on again, you'll know what code is occurring now versus 3 years ago.

Typically, problems with older Toyota transmissions are relayed by flashing the "O/D OFF" light on the instrument panel. However, you don't mention the light being on. My first guess is a faulty temperature sensor or circuit, but usually the transmission won't upshift past 2nd or 3rd, not simply disable the O/D until the proper temperature is detected.

My second guess is the switch itself. If you use it alot, say for driving through very hilly terrain, then the contact may have become worn or oxidized.

I wouldn't wait to change the oil, and post back on what the transmission fluid looks and smells like...this is a good indicator of its condition. Red is good, pink is okay, brown and burned is bad.

Keep feeding info & we'll try to figure it out.