Help!
 

A week ago I got a check engine light and a code that read my engine was running lean. I have checked everything. Spark plugs, fuel line leaks, exhaust leaks. My mechanic did a vacuum test. We ran some fuel line water remover through it everything. But the check engine light is still coming on. When the check engine light came on it was accompanied by a crackling sputtering sound the happens when you accelerate. I thought it was the Mass air flow sensor but that didn't explain the crackling sputtering sound. Like I said we checked the exhaust for leaks and found nothing. The c heck engine light usually comes on after you turn the car off for awhile, then start it back up then it will come on about a quarter of a mile down the road. I desperately need help.

Sounds like a bad o2 sensor.