Yet one more great item for any Commodore 64 owners diagnostic toolkit.
The Dual Diag is a double headed cartridge for the Commodore 64 containing two critical diagnostic tools: Dead Test and the 586220++ Diagnostics.
The Dead Test Diagnostic Cartridge is designed to test the C64 and C128/C128D (in C64 mode), even if your system fails to display video information on Power Up.
The 586220 based diagnostic cart, when paired with a full test harness, the software exercises the CPU, system RAM, ROMs, dedicated ICs (SID, PLA, VIC, CIA, etc) and various I/O circuits of the C64. The test will report if something is bad and possibly display what IC has failed.
Both of these were used by Commodore in house to diagnose issues with Commodore 64s.
You can pick up your very own at Retro Rewind