2003 Hyundai Tiburon Stalling

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.

What are "idling issues"? Idling too fast? Too slow" Erratic? Stalling? What are the symptoms when starting?

There's a number of things to consider. The most common is a vacuum leak. The idle speed system is a controlled vacuum leak around the throttle blade, Another vacuum leak is not controlled, and the Engine Computer will fight to maintain idle speed when there's another leak it has no control over.

The mass air flow sensor can cause this too, and that may be what your mechanic already diagnosed. Also check the fresh air tube between that sensor and the throttle body. There can't be any leaks or cracks in it. That would also let in unmeasured air, and the computer wouldn't know about it so it wouldn't command enough fuel to go with that air. That extra air would be detected by the oxygen sensor in the exhaust system as a lean condition. In response, the computer will try to add fuel or reduce air to correct the mixture. Correcting the fuel / air mixture and correcting idle speed can also be two things in conflict and it can take the computer a few seconds to sort them out.

You need more than spark to fire the cylinder. You also need fuel, compression, and proper timing. Timing is likely not the issue when only one cylinder is involved, but you may want to consider performing a compression test or cylinder leakage test, and you can switch two injectors to see if the misfire moves to a different cylinder.