Carbon monoxide is a very dangerous gas. It is invisible, odourless and very deadly. It is therefore important to check for carbon monoxide when installing or servicing a central heating boiler or hot water heater. This can guarantee the boiler is safe and prevent carbon monoxide poisoning. The Testo 317-3 CO meter. For measuring carbon monoxide in ambient air. It only takes a few seconds to measure. A quick check to see that your working environment is free of carbon monoxide. The Testo 317-3 CO meter measures the carbon monoxide concentration in the ambient air and shows the results on the clear display. The CO meter has a visual and audible alarm that automatically goes off when a threshold limit value is exceeded. The CO meter comes with a carrying case, wrist strap and belt clip so it is always within easy access and the CO meter cannot fall on the ground. The Testo 317-3 comes with a three year guarantee on the CO sensor.