Another software program on your PC is using the serial connection. Close all other programs, or change the COM port assignment number in Windows Device Manager to a low number like COM1 or COM2.
If your AVR-591 is running an older firmware revision, it is highly susceptible to HDCP (High-bandwidth Digital Content Protection) handshake bugs introduced by modern 4K TVs and streaming devices. You can patch this behavior manually by forcing an EDID (Extended Display Identification Data) rewrite. To patch the handshake loop:
If your Denon AVR-591 currently runs without any HDMI switching issues or audio dropouts, a firmware patch is unnecessary. denon avr591 firmware update patched
Compare this number with the latest known version provided by Denon support. How to Update the Denon AVR-591 Firmware
: Requires a physical connection via the receiver's internal service port. Another software program on your PC is using
If your receiver remains unresponsive or continues to display a protection mode light after a microprocessor reset, the internal firmware chip itself may be corrupted. For the AVR-591, official firmware patches are applied via a proprietary service connection on the main board inside the chassis.
[4K Source Device] ───(HDMI Cable)───> [4K Smart TV] │ (Optical Audio Cable) │ ▼ [Denon AVR-591] You can patch this behavior manually by forcing
Release the buttons when the display starts flashing "INITIALIZED."
If you have applied the latest available firmware and are still experiencing issues: