Toyota Repair: Check Engine Light On 2006 Sienna, mile road trip, diagnostic fee

Question
I have a 2006 Canadian Toyota Sienna with approximately 68,000 miles on it (odometer is at 110,000 km). We just got back from a 3000 mile road trip, on the way to our destination our check engine light came on & has been on since (for about the last 2000 miles). I checked the fuel cap and it's on tight & I don't think we purchased any bad gasoline on the trip. I had the transmission flushed for the first time within the past few thousand miles and I checked the level and it looks fine, also I haven't noticed any problem with shifting or acceleration. I mention these things becomes from reading the manual it sounds like a check engine light usually means a problem with transmission, throttle control or emissions. On the way home from our road trip (about a thousand miles after the light first came on) the engine light briefly flashed on and off for about 10 seconds and then went back to being on continuously. It repeated this after another 500 miles or so and has been on continuously ever since. Any ideas? I am hoping it's an emission issue as I would then be inclined to hold off on fixing it as money is tight and we don't have emission testing where we live. I haven't noticed a discernible difference in fuel economy since the light has been going on and have been getting about 24 mpg on the road trip.
Thanks

Answer
The first thing to do is to have the computer scanned to find out why the light is on since there are about 100 different possible reasons, most autoparts stores like Autozone, Kragen, Advance Auto can do this for you at no charge, they will give you the code numbers and an explanation, I only need the numbers so please write these down or get a printout, if the stores I mentioned are not in your area you can have a repair shop or the dealer get them for you but they will have a diagnostic fee. After you have that information for me I can give you a specific answer to what the problem may actually be.