Liftmaster MT5011 commercial operator

From the open position operator starts to close, goes about 6 and then reverses. Door works freely, not hitting anything. Stops haven't been moved, operator was working fine for 2 1/2 years until now. What version is it? Is it a mechanical opener with no circuit board or is it an L2 or L3 Serial # 4907N2261, I think it's mechanical. Sorry serial # means nothing, being mechanical it should have an RPM sensor (and some with circuit boards have them) located inside the control box on one end of the limits shaft. When the clutch slips the rpm sensor will reverse the door. Check to see if the clutch is slipping or disconect the door and just run the opener alone and see if it will go down. If it still will not close, you probably have a faulty RPM sensor. on some models you can unplug the RPM sensor and it will run. Also if you have eye sensors this will reverse the door

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