I have an Accord with 197k miles on it. It goes all the way to red on temp gauge after approx 5 minutes of engine running. I have replaced the thermostat and Radiator Fan switch. I see no obvious leaks and overflow tank shows coolant at maximum. I know both fans work because when I turn ignition to on and turn on A/C they both are spinning. I am really stumped as what to try for next step. Please help!

 

The cooling system has to be bled. There's a bleed screw on the t-stat housing. Make sure the lower radiator hose is getting hot after the t-stat opens and also check to see if you have cabin heat. Then you'll know it's been bled properly.

 

You don't need to know if the fans are working when the ac is on, you need to know if the fans are working when the car starts to overheat.

Also, check that the lower radiator hose is getting hot when the car starts to overheat. If not, it's possible you just need to crack open the bleeder bolt to let the air out as the above poster is suggesting.