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The next step I would take is to test the Voltage of the battery pack with the power on and throttle at zero, and then while keeping an eye on the battery pack Voltage, fully twist the throttle. The battery pack Voltage should start out around 27 or 28 Volts with the throttle released, and should not drop under 24 Volts when the throttle is fully twisted.

If during this test the battery pack Voltage does not drop under 24 Volts then the problem could be with the controller, throttle, or motor. The most likely part to fail in this case is the controller. Throttles usually do not fail unless they are physically damaged, however on rare occasion they do fail on their own.

The motor is the least likely part to fail however testing it by connecting it to a battery is a good way to verify that it works and rule it out as the problem. If the motor tests good then the diagnosis points towards the throttle or controller being faulty. Both of these parts could be replaced at the same time, or the throttle could be tested to narrow down which of these two parts is causing the problem.

The method to test the throttle depends on whether the throttle has indicator lights on it or not. If it has indicator lights then it is a variable speed hall-effect throttle, and if the throttle does not have indicator lights on it then it is a single speed on/off throttle.

Variable speed throttles with lights can be tested using the speed controller's Red, Black, and Green wires that go to the throttle connector, using this method:

Step 1: Test Throttle Output Voltage of Controller

Step 2: Test Signal Voltage of Throttle

If the controller does not output 4.3 through 5 Volts to the throttle then it is either not receiving power, not receiving enough power, or not working properly. If the controller outputs 4.3 through 5 Volts to the throttle then the throttle can be tested using the method of the bottom drawing. The throttle's output Voltage should be around 0.8 Volts at its resting position, and around 4.3 Volts when fully twisted. If the throttle has zero Volts of output, or does not go above 0.8 Volts when fully engaged, then it is faulty and needs to be replaced.

Single speed throttles without lights can be tested by checking continuity between their two orange wires and between their two brown wires. The orange-orange wires and brown-brown wires should have no continuity when the throttle is at its resting position, and have continuity when fully engaged.

Please let us know how it goes.