Abstract: A sewer back up warning system includes a pair of electrically-conductive probe rods fixedly securable within a sewer clean out pipe and being in electrical communication with a sewer back up alarm. An electrically-conductive float is hingedly attached to a free end of one of the rods, such float serving to rise in the clean out pipe as a sewer backs up so as to eventually come into contact with the second rod to thus complete the electrical circuit containing the back up alarm. A corrosion check device is also provided which essentially consists of a manually-operable chain pull for effectively bringing the metallic float into electrical communication with the second rod so as to determine the presence of corrosion build up which would prevent the operation of the alarm.