Looking at the engine from the boat, you will find the sensor on the top left under an insulated plastic cover. You will need a special socket to remove the sensor. A standard O2 sensor socket will not work unless modified. The wall is too thick.
I bought the gasket kit, it has two gaskets that will need to be replaced after removed. If you unbolt the housing that that sensor screws into first, a few open ended wrenches will remove the sensor pretty easy, and then you can get to the sniffer tube to clean that as well. After troubleshooting all last season I found I had one bad O2 sensor, the motor would not hold a steady RPM, new sensor fixed it though.
Andrew: You said you bought a gasket kit that included two gaskets. I didn't see where those go from the write up on the Mako forum. Thank you. Hot Ajax
One is between the block and the part the O2 sensor actually screws into and the other is around the plastic cover. I am pretty sure this is the correct kit, but be sure to check the part number with your model motor. I would imagine they are all the same but never know.
If the motor does not have any symptoms cleaning should be good for them, but as I learned cleaning a sensor that is already bad just makes for a clean bad sensor and doesn't solve anything where I was having symptoms. There is also a harness you can buy that plugs inline to the sensors to check their operation while the motor is running and sensor in place. I put in the generic NKG equivalent and spliced int he wires to determine if that would solve my problems end of last season, which it did. Before I put the boat in I am going to replace it with an OEM and keep the NKG as a spare.
