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Hello! I've been trying to get to the bottom of an issue with a car that is new to me -- bought it in November, and was running excellently until this week. Here is the run through and I'm hoping someone may be able to provide a gameplan for checking systems, sensors etc to diagnose and repair...I want to avoid hopelessly changing parts if possible, and mechanics seem to be scratching heads because it's an intermittent problem.

2010 xc60 t6 awd; 139k miles

Check engine light came on so I took the car to a mechanic who said catalytic converter error code was coming up; but said I could continue to run it / drive it as long as it's running fine...reset CEL. Was hoping to not have to change converter for a bit.

Then, one week later CEL came on for about a day then turned itself off. Car still running fine.

Then a few days later, upon starting, car sounds like a mustang...and, when accelerating, car stutters a bit (but drives) and the loud engine sound comes and goes. Basically, I apply the accelerator smoothly, buts engine responds like I'm flooring it then releasing. Also sounds much louder at times than usual.

Took to mechanic, error code "too lean". CEL was not illuminated.

He didn't diagnose issue because car went back to running as normal and he didn't witness the sound. Reset error codes. Said maybe it had to do with a recent gas fill-up and perhaps the vapor valve from tank got stuck open/closed.

Now, two days later car only runs sounding like a mustang, no smokey exhaust ... Just sounds like it's over revving and rough sounding. Same slight hesitations as before. Worse at low speeds, slightly better/smoother at faster speeds. Sounds louder than a usual engine at times.

Any thoughts where to start??? I'm thinking MAF sensor, vacuum leak, fuel pump, misfire??...or could this all be related to the bad catalytic??? Should I be changing the converter first and then seeing if these issues continue??

I checked a vehicle history and it failed an emission test back in August w previous owner. Just curious how it could have run with no issues for about a month/1,200 miles, then all of the sudden develop this issue.

Thanks in advance for remarks.
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