This is what you pay mechanics to figure out. The sensors only report the condition, not the cause. In this case there's a number of things to look at. There could be a fuel supply problem as in a leaking injector, a leaking fuel pressure regulator, if yours is on the fuel rail on the engine, or a restricted injector. There could be a spark problem resulting in a misfire. A vacuum leak can do this too.
Oxygen sensors do not measure unburned fuel. They only measure oxygen. If that extra oxygen shows up from a vacuum leak, a restricted injector that isn't flowing enough fuel, or the unburned fuel and oxygen from a misfiring cylinder, the Engine Computer is going to respond by commanding more fuel to all of the cylinders on that side of the engine. It doesn't know what is causing the lean condition so it adds fuel to all those cylinders, including the ones that are not running lean. The result is fuel smell at the tail pipe, the engine is running too rich, fuel mileage is down, and the computer continues to see a lean condition.
When the Check Engine light turns on, it means there's a diagnostic fault code set and it refers to something that could adversely affect emissions. Have you checked the codes yet? How did you determine an oxygen sensor is defective, and if it is, why haven't you replaced it?
Sunday, December 29th, 2013 AT 2:56 PM