Today, both radiator fan motors stopped working causing the car to run hot when sitting at an idle. How can I determine if this a relay or radiator fan ? Seems odd that neither fan will come on when A/C on or when the temperature rises to H at idle.

Trace the wire back fron the fan to the relay, check for power to the relay and for power to the switching side of the relay. It's either the fans, relay or the temperature switch.

also check for 12 volts at the terminals that goes to the fans, if you have voltage there then it is definetly the fans, be sure to practice safety while you are doing this because the car will be running and there are moveing parts.

Fan should be running with the ignition switched on and the engine hot even if the engine is not running, so make sure the engine is decently warmed up and don't start the engne.

My nissan altima 3.5 the fan dont ran but when i turn on the ac it running...