Service engine soon light

I have a 99 Tahoe the service engine soon light come on and the dealer told me I needed a canister vent valve. I brought and replaced the part and the light went out then the next day it came back on. The dealer also said that I should change my O2 sensors and don't put high octane gas in my car. Do anyone have any ideas on what I should do? I did not get the code cleared out of the system so I wonder if that is why the light came back on. Please help because the O2 sensors are alot of money. :disappoin

Just go to advance auto and they will tell what codes are currently coming up. They will also clear them for you. On my 96 I keep getting a " Bank 2 Too Rich" code. Nobody can tell me what will fix it. It doesn't hurt the way the Tahoe runs except it puts it into closed loop which send your gas mileage to the toilet. I bought a scanner so that I can just clear the code myself. The first time I had the code cleared it cost 60 bucks. I have had to clear the code about 6 times so the scanner paid for itself. Good luck.