A sensor and alarm system having a power-on sequence. The system has a power supply permanently-like connected to a power line of a facility. There may be a power interrupt to the power supply, which may initiate a power-on sequence of alarms. The sequence of alarms may indicate whether the system is wired properly and operating normally. If the sequence does not occur in a pattern as expected, then there may be a problem with the system. Also, the sensor portion of the system may set off an alarm pattern if a sensed gas has a concentration greater than a first magnitude. The system may set off another alarm pattern if a sensed gas has a concentration greater than a second magnitude. If a sensing element of the sensor portion goes out of calibration, then the system may set off still another alarm pattern.