Different gas detectors detect different gases, so no, not all gas sniffers can sniff out CO. Most portable or handheld detectors are made to sense flammable gases like natural gas so they won’t be able to detect CO. If you need a gas sniffer that can detect CO, you need to check out the specifications on the device you intend to buy and make sure it’s able to detect it (most mountable detectors do sense CO but it’s still necessary to check the specs).