There are two sensors, one on each side of the motor. I was checking the temperature of the top of the head with an infra-red temperature gun. Both sides were about 140 degrees. Unplugging the left sensor turned the horn off. I am going to order a new sensor and gasket. That should fix my problem.
I believe one of the temp sensors is bad on my 1990 Evinrude 90HP outboard. Do I have to pull the head to change the sensor? It sounds the overheat horn, but using an infra-red heat gun, the temperature looks OK.
