Check Engine Light??

I have an '01 Protege. I've noticed the check engine comes on when I start the car and it will stay on the entire duration that it is running. It only has 40,000 miles on it. Now it is due for an oil change, but that wouldn't be the reason the 'check engine light' would come on would it? I've heard the air in the gas tank issue and bad oxygen sensor readings, but are there any other common problems that could be causing the light to come on that I could detect myself? A list of problems/fixes would great. Is the air in the tank issue really a common reason for this occurance? The car seems to be running great and no different than it has since I've owned it. Thanks for the help.

I have an '01 Protege. I've noticed the check engine comes on when I start the car and it will stay on the entire duration that it is running. It only has 40,000 miles on it. Now it is due for an oil change, but that wouldn't be the reason the 'check engine light' would come on would it? I've heard the air in the gas tank issue and bad oxygen sensor readings, but are there any other common problems that could be causing the light to come on that I could detect myself? A list of problems/fixes would great. Is the air in the tank issue really a common reason for this occurance? The car seems to be running great and no different than it has since I've owned it. Thanks for the help.
I have had the same problem ever since I bought my car. My car is a 2001 MP3 and the check engine light has came on at least 10 times. They have changed the sensor for the gas tank and 2 days later is was back on. They are going to purge my gas tank and run some more tests. I will let you know the verdict

What code is given. Can be read for free at auto zone If po421 it is the cats covered under warrenty

The cat was not the problem. I think it was code 721. It was a fuel problem for sure. My bumper to bumper warranty just finished yesterday and the dealership promised they would not leave me hanging. If they do I will see them in court!!

Yeah, that would be awesome. Let me know if you remember. I'm pretty sure it's just a sensor somehwere. No major malfuntion or anything in that nature most likely. My warranty has expired so that's why i'm trying the message forums. Not sure if I trust the auto shops around here. They'll do the diagnostics test cheap, but that will be just the beginning I'm quite sure. Always helped growing up having a certified mechanic and parts store owner as a dad. Thanks for your help ANDYMP3.

I suppose I'll do a little tinkering in the mean time.