Trouble code 134 O2 sensor or MAF?
 

Hello, I have a 1999 V70 Turbo 95,000 miles - automatic, and last week it threw the 134 code. I cleared the code with my scan tool and then drove it for a day or so and it threw it again. While looking through many threads on here I see that this is the front (pre catylist) O2 sensor. I also have read that it could be throwing that code but really be the MAF. What's the best way to know for sure? The car smells like it's running very rich. It also stutters a bit when slowing down for red lights. Any help would be greatly appreciated.