I've been driving an 89 240 wagon for the last few years, and after a few days of rough idling, it suddenly began stalling as soon as it entered idle (it ran fine when I held down the accelerator or manually increased the throttle by reaching into the engine). The only error code was 2-2-1, for O2 sensor; I had it towed to a local mechanic who doesn't have much experience with Volvos, as my regular mechanics were on vacation. Two days later, I find that he's replaced the MAF sensor and the O2 sensor (I had originally suspected that the idle air flow valve was the issue, as it ran fine except while idling). I have a few questions:

1) Would the MAF have shown as an error code when I checked the computer prior to bringing it in to the mechanic? He's done some suspiciously extensive work in the past (replacing the entire master brake cylinder when I asked him to fix the emergency brake).

2) Is it possible for a MAF to be faulty in such a way as to only effect idle?

3) I've had the O2 sensor error since I bought the car in 2007, and I've been getting around 25mpg (automatic transmission), so I've assumed it hasn't been running rich. Was I screwed by that replacement as well?

The car does run well now, but I suspect that it didn't require as extensive repairs as were performed. If you have any thoughts or feedback, thank you so much!