I have had cavaliers in the past and always have problems with the check engine light coming on. It is usually solved by changing the gas cap. Not this time though. The code reads that there is an emissions leak. My mechanic says that I need a vent sensor because it appears to be broken. Are there any reasons that the check engine light would come on?

 

I went through this with my 04 Cavalier and decided to just leave the light on..to turn that stupid light off would probably cost me $100. The car has a vent valve under the car by the gas tank that gets filthy and sometimes that makes the valve stick. A new valve was around $50 more or less. It's also possible it's the valve under the hood which connects to it or any of the lines going under the car. The valve under hood is cheaper but probably is not the problem. I figured I'd probably change them both before I found out which one it was. The dealer had a detector that can tell you where the leak is but of course they charge a bunch to tell you.

 

There is one "wing nut" here. Usually a turn or a twist will solve this.