Originally Posted by Naughton
The SOHC only has 3 of them. Two upstreams, and then a single downstream. I thought it was pretty odd too.
Oh yeah, I forgot about that. On cars with Y-pipes, sometimes there is only one downstream O2 sensor. Oftentimes there is a cat that is behind that sensor too. Why manufacturers put in a cat that isn't monitored, I have no idea.
O2 sensors theoretically last forever, but their real "working life" can be as little as 75-80,000 miles. When they get old, they can start to act up. Usually this sets a code, as the ECM is VERY sensitive in regards to the data it receives from the O2 sensors, but sometimes not.
Also keep in mind that coolant will KILL O2 sensors INSTANTLY. So if you pop a headgasket and coolant gets into the exhaust, plan on replacing all the O2 sensors too.
I had this issue with a '92 Sable that I did headgaskets on (common issue with that model year). Car had intermittent and random misfire issue with a CEL while driving. Replacing the O2s fixed that.