Oh the irony! Same thing happened to me today. I have an '01 manual wih A/C. Today the heat index was 103, tommorrow 106. I went to a local shop and was told the A/C cancels out to relieve stress on the engine. Then I blasted full heat on the floor to bring the gauge down. Am going to the dealership in the morning. local shop told tales of Nissan owners running into headgasket problems, but I am praying I will only have to pay for a radiator due to blockage. Any idea what causes this? Any prevention? If not, any idea how for headgasket? Thanks for the URL for the radiator barn.