The manual says it thinks there is a short circuit on the amplifier output side. If it still says Protect after disconnncting all the speakers wires there is an internal problem. Take it to a hands-on tech.
If the problem disappears, you had a short. Attach speakers one at a time to isolate the cause.
http://www.retrevo.com/support/Sony-STR-DE345-Receivers-manual/id/387bh943/t/2/
If you suspect that one or more of your speakers are a problem, simply push the speaker switch to turn them off. Turn on the power to retest. Bad speakers are NOT the usual cause of this problem.
Typically there is either a short in the outputs, or at the very least an imbalance in the gains of the driver transistors in one of the channels, this would cause over-conduction thus triggering the protection circuitry. Other problems can be bad solder joints anywhere in the power supply, driver circuitry or on or around the several voltage regulators in these Sony receivers. I've worked on a number of these and have found them to be very prone to bad solder connections anywhere on the main controller board to the amp board, take your choice. To repair you'll need a competent tech in a shop. If you want to download a manual for this model, go here: http://www.manualslib.com/download/162899/Sony-Str-De245-De345.html
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