Check engine light went on Monday evening when I started my 2004 Hyundai Elantra. I shut it off and tightened the gas cap. That didn't work. It is still on as of Tuesday afternoon.

You had a starter replaced,paid for that,
even though nothing works,not even
the new starter?
Where did you go for that work?
AAA- didn't know they diagnose vehicles
& give advice,you been had by two parties

You got ripped off by the mechanic. A good mechanic can pinpoint the problem by using a scan tool. Take vehicle to autozone or similar and have it scanned for codes this will give you a idea what is wrong. I believe the problem is exhaust related because of the blinking light.

most of the time a engine light will set with an Evap leak code if the gass cap is not tight or left off. tighten cap, or replace cap if gasket is cracked or worn.

Hello, have you tried to unhook the negative side of the battery for about 10 minutes and then go from there . It should reset the computer completely .

Sounds like you have done a lot of the right work here. I popped that code on my 2001 Elantra within the first 50k miles. Turns out that in my case the gas cap was the culprit. Keep reading though, maybe this still applies. My cap was still very new, and it's not that I forgot to put it on or to tighten it. The mechanic at the Hyundai dealership said this happened all the time to this car, and that you simply have to get the cap tighter. It has that ratcheting release so as to not over-tighten it, but here's the trick: Tighten as normal, allow a few clicks for good measure. Then, press in firmly against the cap and you'll see that you can get another 1/8 or so turn out of the cap. I popped this code 3 times total, the first time the dealer just reset the light. The second time was when he told me about this, and the last time was accidental on my wife's part. :) You can reset the light by simply removing the battery terminal... 30 minutes seemed to work for me... and then reattach. Of course it goes without saying that this is with the car off. Good luck!

I'm not that user, but go to Autozone and have them scan your computer for the code to your check engine light. It will point you to which sensor to replace, as this is likely your culprit. There are two speed sensors which are critical to transmission shifting on this car. The one I'm familiar with is on the passenger drive axle, but when mine went out (I have a 2001 Elantra as well) I didn't have any of these issues you describe. I don't know where the other sensor is, but that's where I'd suggest starting. Get it fixed quickly though, the web-boards I've read warn that these symptoms can be pretty ******* this little transmission. Good luck!

Start here... tighten your gas cap till it clicks. Then lean into it with added pressure and tighten further... you should be able to get at least 1/8 turn or more. Then, disconnect your battery terminal (with car off of course) and let sit for 30 minutes. Re-connect terminal, start car. Light should be reset now. If it stays of, there's your problem. If not, the problem is much more involved and you may want to consider a mechanic. This is the free fix that seems to work much of the time. Obviously it just means your gas cap is not sealing well. Mine started doing this at about 50k, I'm over 125k now with no issues. I just have to remember to get that extra tightness every time and no problems. Good luck!

tightening your gas cap can sometimes fix this problem. Think if the light came on after buying gas.
If not it's usually the rubber seal around the gas cap. Just measure and replace with a similar rubber seal available at any hardware/auto parts store.

some times light wont reset self got to have it shut off autozone want shut it off but most advance auto will or disconect pos. battery cable for 30 min. it will normaly reset light

Yep. As silly as it sounds, that's correct.