Focus Overheats when A/C is off

Please Help. My Focus is overheating while in traffic. When I put the a/c on the temp gauge goes back to normal. It seems that the two fans are working with the a/c but the single fan won’t kick on when the a/c is off. I have gone to extremes getting sensors for this Focus and to no resolve. It has been pressure tested and has no leaks. The coolant is freshly changed along with the water pump and hoses. It is a huge expense to bring the car to the dealer. They now want change the entire fan assembly to the tune of 748.00 dollars. What is that all about? There has to be a cheaper answer. What happened to the day when you changed the thermostat and it worked?


Thank you, John