1. Go through startup and observer all the motors, try to pinpoint the one which behaves differently.
2. Read the flight log. Search for the ESC related messages. Find number of the ESC which has different messages than others (ie. lacks some of the messages, or simply reports error).
If you've pinpoint the ESC number, you can easily locate corresponding motor: ESC0 -> M1 ... ESC3 -> M4.
unfortunately the log just spits out a generic esc error and doesn't tell me which is faulting:
2.910 : 2508 : 0 [hardfault]:******************check fault info and trace ************
2.910 : 2508 : 0 [hardfault]:-----fault is null: addr(0x40024000),flag(0x842ed686)-----
2.910 : 2508 : 0 [hardfault]:-----fault is null: addr(0x400241a0),flag(0x87ba5e73)-----
2.910 : 2508 : 0 [hardfault]:-----task info is null: addr(0x40024340),flag(0x63e944c5)-----
2.910 : 2508 : 0 [hardfault]:-----trace info is null: addr(0x400244d0),flag(0x626ad5c3)-----
2.910 : 2508 : 0 [hardfault]:******************check last trace ******************
2.910 : 2508 : 0 [hardfault]:-----trace info is null: addr(0x40024790),flag(0xa1094ab2)-----
2.910 : 2508 : 0 [hardfault]:-----wdg_time_info is null: addr(0x40024a50),flag(0x99c96dac)-----
3.762 : 3019 : 42 ESC0 link up
3.773 : 3026 : 43 ESC1 link up
3.785 : 3033 : 43 ESC2 link up
3.797 : 3040 : 44 ESC3 link up
3.797 : 3040 : 44 esc alive info = 0xf
4.760 : 3618 : 92 ESC0 version: Protocol = [V1.0] Hardware = "WM320_ESC_V9"
4.760 : 3618 : 92 Loader = [V01.00.02.02]
4.760 : 3618 : 92 Firmware = [V01.11.00.00]
4.768 : 3623 : 92 ESC1 version: Protocol = [V1.0] Hardware = "WM320_ESC_V9"
4.768 : 3623 : 92 Loader = [V01.00.02.02]
4.768 : 3623 : 92 Firmware = [V01.11.00.00]
4.777 : 3628 : 93 ESC2 version: Protocol = [V1.0] Hardware = "WM320_ESC_V9"
4.777 : 3628 : 93 Loader = [V01.00.02.02]
4.777 : 3628 : 93 Firmware = [V01.11.00.00]
4.785 : 3633 : 93 ESC3 version: Protocol = [V1.0] Hardware = "WM320_ESC_V9"
4.785 : 3633 : 93 Loader = [V01.00.02.02]
4.785 : 3633 : 93 Firmware = [V01.11.00.00]
49.210 : 30288 : 2315 [M.Start]REQ_RC_NORMAL FAIL, esc error!
58.810 : 36048 : 2795 [Ctrl<6>] REQ_APP_WM610 ENGINE_START ctrl_engine_start
58.830 : 36060 : 2796 [M.Start]ACT.Takeoff FAIL, esc error!
58.850 : 36072 : 2797 [M.Start]ACT.Takeoff FAIL, esc error!
58.870 : 36084 : 2798 [M.Start]ACT.Takeoff FAIL, esc error!
58.890 : 36096 : 2799 [M.Start]ACT.Takeoff FAIL, esc error!
58.910 : 36108 : 2800 [M.Start]ACT.Takeoff FAIL, esc error!
58.930 : 36120 : 2801 [M.Start]ACT.Takeoff FAIL, esc error!
58.950 : 36132 : 2802 [M.Start]ACT.Takeoff FAIL, esc error!
58.970 : 36144 : 2803 [M.Start]ACT.Takeoff FAIL, esc error!
58.990 : 36156 : 2804 [M.Start]ACT.Takeoff FAIL, esc error!
59.010 : 36168 : 2805 [M.Start]ACT.Takeoff FAIL, esc error!
59.010 : 36168 : 2805 CTRL reset all by auto takeoff start fail
59.010 : 36168 : 2805 try auto takeoff, but start motor fail!
59.030 : 36180 : 2806 [M.Start]ACT.Takeoff FAIL, esc error!
59.030 : 36180 : 2806 [Ctrl<1>] REQ_RC_NORMAL ATTI_HOLD ctrl_gps_atti