This is an infrared temperature sensor, which enables you to determine a dead cylinder from a live one based on exhaust manifold or header tube temperature. It can also tell you a lot about coolant temperature. Check upper and lower radiator hose temperature. If you find the upper hose considerably hotter than the lower, your radiator could be blocked. Harbor Freight can set you up with its Cen-Tech Infrared Thermometer, which is a great troubleshooting tool for engine tuning and cooling problems.