Your mechanic knows more about what's going on than we do. First you have to understand that diagnostic fault codes never say to replace parts or that they're bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operation condition. Second, there's dozens of fault codes related to the oxygen sensors and they mean very different things. We need to know the exact code number to determine a plan of action. As far as the mass air flow sensor and whether you believe him or not, he must have a reason for his diagnosis. The proof is if the problems are solved after the service.
Running too lean can cause a single-cylinder misfire due to insufficient fuel, but a defective spark plug can cause a lean condition to be detected too. That's not because the fuel / air mixture is lean. It's because along with the unburned fuel from the misfiring cylinder, there's unburned oxygen too, and that's what oxygen sensors detect.
I would judge your mechanic's diagnosis by how much time he spent on your car. If all he did was listen to the complaint and form an opinion, I'd be suspicious. If he read the fault codes but didn't do any followup testing, I'd hope he was using his experience to back up his recommendation. If he already tried the more common suspects and found the problem is more involved, I'd be more inclined to let him continue.
Monday, October 13th, 2014 AT 3:25 PM