99 Avalon, engine turns, but won't fire, need help!

I recently bought a used 99 avalon. It drove fine for a couple of days, then it wouldn't start. The engine turns over...but won't fire. I have been advised to check my fuel relay, that's new and works fine...I also checked my fuel pump...and that works fine. I was lead to believe it was a fuel problem by trying to start the car with starting fluid. but now, i am at a loss for what the problem could be. i have taken it to a shop, where i trust the mechanic. however, he is also at a loss. I have only had the car for a couple of weeks, and it only ran for 3 days...I'm naive when it comes to vehicles and need help! any ideas?

Check your ECM for stored codes with a scanner.

Check the spark plugs for fouling and appearance - particularly the hard-to-get-to ones.

Check the resistance of the spark plug wires.

Check the compression of all the cylinders.

Check for leaks in the intake system

Is it safe to assume the mechanic made sure the timing belt was intact? If it's broken the engine will crank all day - the bottom half at least. Just won't start.

After my "new" car had spent two weeks in the shop, the mechanic came to a bright idea. We had suspicions before about the anti-theft system and that is what the problem was. The key that we were using to try and start the car, had a bad memory chip in it therefore, it was denied by the security system. After buying and trying all new parts, a new key was all that was needed. So, now I have a large bill to pay the mechanic, and although I am relieved it was a small fix, I do feel stupid. Thank you for all of your help and suggestions! Hopefully now my car won't need new parts for quite some time!

Thanks again,
Brittan

That was a real deal. Not many mechanics would try replacing a key. That guy deserves a lot of credit.