I have a Yuneec H480 that I bought a few years ago very cheaply because it got into 5 motor mode occasionally. I tried replacing the motors not working but with limited success.
Recently I opened the drone thinking I would replace the mother board and see if this fixes the issue. During disassembly I found, so far, three dry solder joints on the red/blue/black power feed to the engines. Two on the mother board (blue wires) and one on one the male contact on a wire from the motor (I don't remember the color). The interesting thing is that the multi strand wires look absolutely "dry" with almost no traces of solder.
I have fixed the solder joints and when the battery is charged I will do a test flight again
. If I get a new five motor fault again I will re-do all motor power connections. A power feed problem is very easy to feel by rotating the motor. If one or more power feed is disconnected the motor turns extremely smoothly. When the power connectors work the motor works as a generator and turning it one feels the breaking force from the windings.
Notice that fault finding as such is fun so even if I haven't done much flying with this specific drone the fault finding has been interesting. I have mostly flown Q500 and like it very much.
After the long intro above a question:
Has anybody else seen similar "dry" solder joints? I have a feeling that the reason for the problem could be that the previous owner possibly stored the Drone in a cold shed during a few winters. We are talking about Finland and a possible storage temperature of -30 deg C (-22 F) or below for a few months. Today's solder is essentially pure Tin which means that in cold temperatures below 13 deg C (55 F) the Tin structure changes from metallic to non conducting powder. Combined with vibrations this could explain the observed "dry" solder joints?
Has somebody else seen a similar problem possibly related to low storage temperature?
Test flight:
The test flight result was slightly mixed. I hovered for perhaps 10 minutes without any problem. Starting moving the drone more aggressively caused 5 rotor mode but landing without problems. After re-starting the drone I wasn't able to reproduce the error. My feeling is that I have one or more additional similar problems where I higher power load triggers 5 motor mode possibly because additional solder points are marginal for the power required. Probably it is a good idea to re-do all the motor connections ... when I get enough red/blue/black heat shrink tubes
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? The program reported the ESC as OK. Testing the different motors resulting in all working but the left hand front motor (when looking from the back towards the front) has a clearly different sound than the other motors. Doing a test flight a few days ago, see this thread, caused a 5-motor fault. Looking at the flight log I found the motor status going from 255 (OK I guess) to 254 which I guess is motor number one which I assume is the front left hand side motor with the different sound. The relevant line in the log is I think shown below. The motor status stayed 254 until landing and switch off. 
