[jdmccright]

There's an unwritten rule among repair shops that O2 sensors should be changed every 10 years or 100k miles. For older vehicles, they aren't that expensive so it is typically not a bad deal. But the newer ones can run up to $150 each...with 3 or four sensors per vehicle...you do the math. Unless you are experiencing poor fuel mileage that is not explained by any other malfunction, then you might consider having them checked.

Time & tools are all you need to replace the timing belt. You should also consider replacing the water pump, tensioner and idler pulleys, and all the oil seals. Refer to the stickied threads at the top of the forum for your car (a Gen IV Camry V6 is similar in this instance).

If you bought all the recommended parts plus the tools you'd need, you would still save about $150, AND you'll have the tools and knowledge to do it again.

Good luck!