I have a 2002 Toyota Tacoma with close to 97,000 miles on it. The other day the Check Engine Light came on, so I took it into the Dealer Service Center. They said it would be $110 dollars to run the diagnostics, to be applied towards any repairs that would be needed. Two hours later they call to tell me the error code was in the evaporative(?) system, They ran checks on whatever one checks, and everything was okay, so "it was probably a loose gas cap". They said they tightened the gas cap, reset the codes and drove it to make sure the light didn't come on again. Now I can't really begrudge them the $110, because they did have to run all of the tests to make sure it wasn't something serious, but it does bring up some questions. If this were to happen again in the future, how safe is it to just double check the gas cap and keep driving? Which raises other questions. How long would it take to know if this really was the problem and it is fixed? (Assuming the cap was left loose at the last fill-up, it took about 10 days and a quarter tank of gas before the problem was even detected.) Will the CEL turn itself off, or does it have to be reset somehow? Basically, if I tighten the gas cap will I get enough feedback to know that was indeed the problem, and how much leeway do I have (hours?, days?) before I know it worked or that I should go ahead and have it checked out?