I have an old Otis job that was upgraded around 1999 with MCE controls. Two car group, eight stop traction, I believe 350fpm, MAC door operators and hoistway door gear. One of the cars intermittently shuts down at the lobby, doors closed and often traps passengers. The building engineers often mainline the car to free the trapped passenger, normally this really bothers me. At this building in particular I understand, they house recovering addicts and mentally insane individuals. I arrived at the building today, car shut down with nobody trapped. The LEDs on the flip down unit indicated "leveling sensor failure" (inactive). A fellow mechanic told me that these faults often indicate door lock issues. I burnished the shorting bar, checked pick up rollers and cleaned the door track at the problem floor. Is there anything else that I should be looking at?