Car amplifier protection mode Sony Xplod 600w xm-554zr

Protection mode on means something is shorting somewhere or bad ground.

Used amps have contacts coming off the motherboard, look and see if those are solid and not moving. If moving those will require soldering.

If your speakers are connected, then disconnect them and try again.
If no more protection mode then problem is Speaker or speaker Wiring, make sure cables don’t touch each other unless your connect same type of speakers in Series ( - goes to + , in this case your resistance increases in sum ),
or Parallel ( - to - and + to + , then sum of resistance divides by 2).

If your protection is still on without speakers connected then look for burnt elements, if there are none like in my case, then check resistance from speaker connectors to transistors shown in the video.

My transistor got burnt from connecting to high power input or bridged wrong resistance.

Hope I’ve saved you some time.