The ATR 42-500 veered off runway 09 during landing at Coari-Urucu Airport, AM, Brazil. There were no injuries, but the aircraft sustained substantial damage. After approximately one hour and ten minutes of flight time, during the descent procedure, the no.2 propeller rpm suddenly increased. The crewmembers performed the procedures set out in the aircraft manuals and proceeded to their destination. The crew carried out an NDB approach to runway 09 at Coari-Urucu Airport in light rain conditions. After touchdown the right propeller did not assume the reverse pitch with the throttle in position below Ground Idle. The aircraft then yawed to the left and veered off the runway approximately 500 m after the touchdown point, traveling about 180 m through the grass. Contributing factors. – Attention – a contributor. The lack of attention to the lighting of the Low Pitch lights contributed to the application of reverse in both engines, with only one of the lights on. Thus, this action contributed to the asymmetric reverse condition, which allowed the aircraft to exit the runway on the left. – Emotional state – undetermined. The feeling of apprehension experienced by the crewmembers in relation to the length of the runway, associated with the condition of night operation and in the rain, may have affected their performance, so that, in an attempt to anticipate the engines entering the reverse regime, the standard landing procedures were not properly performed. – Aircraft maintenance – undetermined. The series of discrepancies identified in the Microswitch Box, PN 11S76180650-10, SN 0199, indicating that the unit could have been opened in an unauthorized location, raised doubts about the quality standard of the services performed on the aircraft, which may have allowed an aircraft grounding condition fault that has affected several aircraft electrical components and caused the Np Cancel function to fail.