Fan on full speed problem

I have a Mac Pro 2006 Quad core with 2 x 3.0Ghz Zeon processors. The front and rear fans are on full speed and the Istats show the CPU 1 temperature at 26 degrees, CPU 2 Temp at 126 degrees. To try and identify the problem I first substituted the CPU 2 cooler wih the sensors with the CPU 1 cooler, but it made no difference. I did the same with the processor and that made no difference. The temp on CPU 2 still reads 126 degrees and the fans full speed. I upgraded the OS to 10.7.5, still no difference. Has anyone any suggestions please? Thanks Stephen