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New customer and CO alarm going off with new batteries installed. My Sensorcon personal CO detector registered 0 ppm when entering the home and stayed there, as did my Bacharach Monoxor. I did a combustion analysis on both the water heater and furnace and CO was less than 20 ppm on both. B-vent was unobstructed and venting well, all visible joints solidly connected. No car started in the garage, dryer or stove weren't in use, and no fireplace. Advised her to get a new low level CO alarm.