Boot using Apple diagnostic Tools, the same disk for resetting the fans and you can do processor tests and it will tell you which one is toast (if you get it to boot).  My dual 2.0 would not boot and it was the main proc that was faulty.  I switched them, let the fans run on high and confirmed.  Then bought a new set ( one of those was dead) which was ok as I only needed 1.  Then reset the fans and was rock solid ever since.