SID unit power supply: Diagnostics
If no current from the +30 circuit is present, the display will not be active and diagnostic communication will not be possible.
If no current from the +15 circuit is present, the display will be off but can be activated via the bus, by turning on the radio for example. Diagnostic communication is possible with limited functions if the ignition is turned ON.
The SID unit will not generate a diagnostic trouble code if no current from either the +30 or +15 circuits is present.