One of four motors of my P4 is stiff. It is located on the lower left if you look at the drone with nose up and battery down.
By stiff I mean the following: I can rotate each motor with my hand. Three of them rotate very easily with some faint soft internal "clicks". The fourth one also rotates, but I need considerably more force to get it from "click" to "click". I.e. I force it out of some fixed position and then it jumps into the next fixed position, etc.

As much as I can tell, this does not affect flying. Battery life is ok, stability in air is ok and the motor temperature is ok.
But I don't like the fact that one of the motors is behaving differently. I had an idea to replace it with a brand new motor. Can it be done at home?

As for the other questions - have finally flown my P4 outdoors. I would say everything went well (especially RTH - is precise down to couple centimeters!), except some strange events that happened time to time.

1. I took the drone to the open rocky space far from any city with no buildings, no power lines etc. (I attached the photo of it.) But it made me calibrate compass twice. It didn't even calibrate from the first attempt.

2. Both distance and height seem to be very limited. At only 50 m height or 500 m distance I was getting "signal weak" warning. Again, the place was fully open with direct visibility in all directions (sea beach).

3. Once after landing, it displayed "vision system error" which disappeared only after I rebooted the drone.

4. The left stick is very sensitive so when I get the drone up it also begins to rotate because I tilt the left stick for a very little amount but that's enough already. Can't keep it straight up.

I'd like to hear some advice or explanations if you experienced the same things.

Hello dr_noise,
1.Please try to lift the drone up and do the compass calibration again.
2.Adjust the antenna orientation when you get the "signal weak" warning.Please see the video tutorial below.

3.If the "vision system error" appear again, please try to calibrate the vision system via the latest DJI Assistant 2 in PC.
Here is the tutorial:
https://www.djivideos.com/video_play/57301fc0-978b-4d7d-afa8-3d4082064a81
4.For the sensitive issue,you can enter the Main Controller Settings- Advanced Settings to set up the EXP and Sensitivity.

"4. The left stick is very sensitive so when I get the drone up it also begins to rotate because I tilt the left stick for a very little amount but that's enough already. Can't keep it straight up."

I wonder if the RC needs calibrating ?

I would take it home and do all calibrations visual Sensors (As per DJI-Mindys suggestion) IMU  (on a level surface), Gimbal, RC & Compass. When doing compass make sure there are no metallic objects close by (specifically ferrous (iron, steel)). And leave the remote about 10 feet away.

In my opinion, Calibration has nothing to do with the physical ability of the motors to spin freely when the phantom is turned off.
The bottom line here is that you consider one motor significantly stiffer (harder to turn) than the others.
This is abnormal and I would consider this a problem.

All motors should spin freely and equally.    It is possible that at some point a long strand of hair became wrapped around the shaft.   Or there could be an internal problem with the motor itself.
In any event, if you are easily detecting a significant difference in the ability of one motor to spin compared to the others I would have that corrected.

Another way you can determine the extent of the problem is by feeling the motors immediately after a flight or by taking their temperature with a laser thermometer.   If that motor (or any motor) is significantly warmer than the others, or if any motor is hot to the touch while the others are not, that might indicate a motor with too much internal resistance.   If the temperatures are all about the same after flight, then I would not worry too much.    I would just check it periodically.  What could cause that condition?   If one or more of the magnets was slightly closer to the armature than the others this could occur.

Yes, you can replace a motor yourself.   But if it has warranty, and you can send it to an authorized repair center, that is better imo.

Thanks all! The antenna orientation tutorial was really helpful. I did not test it yet but I think my problem with weak signal is exactly that. Signal weak while ascending - the drone was directly above me. Signal weak while flying away - the drone was to the left of me. In both cases antenna orientation was far from optimal.

Also I'll do all the calibrations.

About the motor - I doubt my drone has a warranty. I bought it from Ebay seller, it looks like new but it was obviously used and does not have any purchase documents. Also there is no authorized DJI dealer around so I'd have to send it abroad for the fix. This can take enormous amounts of time and the shipping itself is also very expensive. So I'd like to try to replace the motor myself. How hard is that?

Aardvark Posted at 2016-8-30 12:28
"4. The left stick is very sensitive so when I get the drone up it also begins to rotate because I t ...

I don't think RC needs calibration. I think my technique is wrong. I push the L stick up using my thumb placed to the end of the stick. That way it is hard to maintain strightly vertical position. I think I need to push the stick up by placing my thumb under the stick, not on top of it. I've seen such technique in some videos.

dr_noise@mail.r Posted at 2016-8-30 17:11
I don't think RC needs calibration. I think my technique is wrong. I push the L stick up using my  ...

Good one, I never really considered that. I usually have my thumbs pushing side of sticks.

Hi, just wondering if you ever got to the bottom of your stiff motor issue or did you bother getting it fixed and if it ever affected the drone in the end?
I just got a mavic air 2 and I'm having this issue but like you it's not effecting anything in flight. Only when I stop the motors I notice. One stops straight away. The rest gradually slow down. No warnings or anything in flight.. but I can also feel it when i turn them with my fingers. Its minimal but definitely there.

Thanks in advance