Can you guys help me locate it (Merc 7.4), think I need to replace it. The boats a 2000 312 Formula Fastech.

are you looking for the guage sender or the alarm sender,they are 2 seperate senders.

The gauge sender is down below the #7 spark plug and screwed into the block.

Thanks Griff,

Is it hard to replace?

That's correct.

The gauge sensor will have a light blue wire going to it. The alarm sensor is the smaller of the two, and has a tan wire with a light blue stripe going to it.

Pretty simple as long you can get to it. It just screws into the brass fitting in the block and has one wire going to it. Do not use a sealer on the threads or the sensor will not ground.