Sony Receiver protect code 21 - Audio Players & Recorders

Protect is the special diagnostic mode that starts when the receiver detects an error. Errors are triggered by a lose contact, a short, a contact to ground a blown component, bad or incorrect speakers or a bad connection cable.
Code 21 means DC detection circuit error. This is often started when the speakers are connected. Or if you use a bad video cable.

Ensure that speakers are properly connected and that you are using the right speakers.

Try first removing all other cables apart speakers connection, and unplug power cord fro ten minutes. If it is just the speakers this will clear the error code.

If the error code does not clear by unplugging , then there is either a faulty speaker wiring, or an internal fault, often a short to ground on the input audio connection board.