The Yellow Jacket 69371 is a replacement sensor for Yellow Jacket 69373 Combustible Gas Detectors.1 Replacement sensor. Unit is preset at normal sensitivity, but you can switch to high or low. Slow beeping indicates that unit is warmed up. Frequency increases when a leak is detected until an alarm sounds when moving into high gas concentration. The SmartAlarm LED indicates leak size. If no leak is detected in a suspected area, select high sensitivity. This will detect even low levels in the area. Move the tip over more defined areas, and you will be alerted when the tip encounters the concentration at the leak source. With ultra-sensitive, long life sensor. Detects all hydrocarbon and other combustible gases including propane, methane, butane, industrial solvents and more.