How do you reset the check engine light on a 2001 Toyota Avalon?

First, if it is something like a loose gas cap, just tighten it and it will reset itself in the next 100 miles or so. If it is something else, like a leaky gas cap it will never reset. But to reset the OnBoardDiagnostic (OBD)Computer... Go to eBay and search "obdII" and buy one, they run about $50.00 including shipping. There are two types, ISO for Chrysler and some Japanese, and the other one for all the others like Ford and GM. There are several that are both, that is the kind you want. Be sure to email the eBay seller and ask if it is used with your year and model. (1996 was a changeover year for OBDII technology, so this advice may not apply to everybody--but the dealer service guy can tell you over the phone--ask him if it is OBDII compliant.) When you get it, you just plug it in and let it cycle 4 or 5 times and it resets everything. You then have to drive your car for 50 to 100 miles for the onboard computer to test everything, and in the meantime your car will fail smog checks because it has no history in the computer and the smog test procedure demands a history even if there is no pollution. Cut and Paste http://search.ebay.com/OBDII for a list of them for sale on eBay. Most units come with a list of generic fault codes so you can tell what the problem is, this could be very helpful.