My marantz SR 7400 suddenly shuts down or goes into protect mode shows code CKPWR5

Most likely you have a speaker short, you need to check the ohms on each speaker disconnected from receiver if they are 8 ohms they should be close to that on the meter no more than a 1 and half ohm varience to what the speaker ohms says on the back. If they are ok you need to disconnect speaker wire from receiver and check for a short in the wire by connecting ohm meter leads to ends of the speaker wire you should get no reading if the wire is good. If this is all good you have a bad output transister in your receiver, but i dout this is the case because these transisters normally short to ground and blow a fuse. Hope you solve your problem good luck