FIREBOY M-2A-R The M-2A is a simple, yet highly effective detector of engine compartment fuel vapors, as well as of fumes (unburned hydrocarbons) from faulty exhaust systems and hydrogen (battery vapors). When any of these fumes are detected, the M-2A is immediately activated. A visual warning light illuminates and the audible alarm sounds. The sensor is engineered to alarm at 16%-20% of the Lower Explosive Limit (L. E. L. ). PRESS TO TEST for each sensor confirms proper function. For an additional senor use order# 141249 - FBY-MS-2.