in my case bafang was quick to answer but dengfu was the one who is late in replying. bafang even set up a group email with dengfu and me to resolve the warranty process. in the end they eventually replace me a new motor.
Check all wires thatbitbis properly connected. Just try to insert properly. Sometimes it is caused by cables not fully inserted specially the power cable. Hope this helps.
Hard to claim warranty from dengfu. They said bafang hasn't replied to them yet for almost 3 weeks now. But when I emailed bafang, they immediately reply. My m600 is still defective.