when the check engine light comes on, there is a problem and it can be solved by looking under the carpet located at the foot of the passenger side. pull the carpet back and you will see a "screen" with a blinking light. count the number of constant blinks (some times there are multiple codes). then check here for the corresponding number. the O2 sensor can be bought at autozone for approx. $25, and changed out with a tool supplied by them ($15 renters fee, returnable on return of tool). took me 20 min tops to do both front and back. cheap and simple and gas mileage will surely improve. good luck.

Ike