When the ignition is switched on, the system voltage is applied to terminal A1 of the SDM from the IGN 1 fuse in the UHFB. The SDM self-checks the system while the I/P cluster performs a bulb check by flashing the air bag telltale seven times. If a diagnostic trouble code is detected, the SDM will command the I/P cluster to light the air bag telltale when the bulb check is complete. If no DTC is detected, the telltale will not light. The SDM communicates with the I/P cluster on circuit 1807.