2004 BF150 - Where is the Air Fuel Sensor

I was getting a constant loud beep off my port engine, and plugged in my ODB software tool which told me that I had an A/F Sensor failure and A/F Sensor Heater Failure.

I'm assuming that AF stands for air fuel.

I found the relay and reproduce that alarm on my other engine by disconnecting it on my other engine, but disconnected all kinds of other sensors and never found the other one.

I'm starting to think it's buried somewhere in the exhaust manifold but why would they call it an air flow sensor if it was in the exhaust?

Can anybody help me out with this? I found the part for about two hundred bucks, but don't know where to put it. :

It's an O2 sensor, screws into the exhaust manifold. Not common on a 150, could be other issues