The D420A has an A and B key, the A key is programmed to perform Command 44 (Local System Test) and the B key is programmed to perform Command 47 (Reset Sensors). The D620 and D636A also have a C key which is programmed to perform Command 5. Command 44 and 47 are enabled by default and can be disabled in the System Test/Reset module by selecting No in the related parameter. Command 5 is disabled by default but can be enabled by selecting Yes in the Cmd5 PassChg parameter in the Command Options Module in programming.