How do I check to see if the temp alarm is working properly?? The impeller went out, and I never got an alarm?? The alarm sounds when the key is turned on though? Thanks for your time, and help.

The overheat alarm point on a stock 496HO is about 190 degrees F. If the engine did not get to that temperature the overheat alarm code may not have been sent by the ECM. In any event if you lost an impeller or such you normally would have lost water pressure and the low water pressure code alarm should have set off the alarm. I would recommend hooking up a scan tool to the engine and checking for active fault codes as well as alram codes shown in history and see what is there. If no codes are showing in history then I would say something is either wrong with the ECM or sensor circut. Has your ECM had any reflashing or reprogramming done to it. I know with some current reprogramming being done by others in the industry some of the guardian alarm features can be turned off and will not throw alarm codes.

Best Regards,
Ray @ Raylar