I have a 1998 Toyota Avalon XL - the engine sign is on for a while...the mechanic says it will cost $300 - it's the oxygen sensor...is it right?(He says that after replacing I have to do 50 miles...then the light will go off).. What if engine light will come back in let say 2 months...

If the sensor is bad and needs to be replaced, the light needs to be reset at the time of repair.

If your mechanic does not have the equipment to reset the light, he can not accurately diagnose the light either and is likely just guessing.

Find a new mechanic, this guy smells like trouble.

O2 sensors do cost $100 or more.your computer should re-set [light off] after a period of operation with no faults.there are many things that will set off a trouble code in the computer.the light signals you it has stored a code.