I have a merc 525 and have a Gaffrig Monster racing water temp gauge not working. I ordered a replacement 12006 replacement which looks the same but the temp reading is off by 40 plus degrees. I ran it on a smartcraft diagnostic meter and the motor is running at the correct temp. So my question is am I supposed to change the stock merc temp sender to a different one and if so which one or part number.
PS: Gaffrigs customer service sucks. They wont talk to you on the phone and insist on you e-mailing them your question. I e-mailed them on Monday and again yesterday and no response yet. Now I have to come on here and ask you guy's LOL

Tale a hand held temp gun and measure temp at the temp sender to see what it is.

Next info is the temp gauges are OHM specific range so the sender has to to match that range.

Sounds like they sent you on oil temp sender. What is the part # they sent?

I have the same problem with a Autometer gauge, the ecm sees the right temp but i have it narrowed down to a bad ground somewhere. The ohm readings are correct at the sender but by the time it gets up to the gauge it's off. Mines an issue with where the gauge gets it's ground from. Gaffrig should be able to give you the ohm readings at a specified temp and you can compare it to the sender you have in the boat.