Service Engine Soon Indicator
WARNING: This page is about a different car, the 2003 Mazda Tribute. However, it is still accessible from the selected car via links, so may be relevant.
When the ignition is in the ON position the bulb will prove out. During vehicle operation, the malfunction indicator lamp (MIL) illuminates when the PCM has detected a fault. If a fault is detected the PCM sends a signal to the instrument cluster via the SCP (J1850) line and turns on the MIL lamp.