I have a 2 speed spa motor (K48m2a4) that stopped working at low speed. going to high speed only has circulating power.

This is the only diagram available, but it does show high and low pump contacts.

https://trinityelectroniclabs.com/wp-content/uploads/2018/09/APOLLO-11-DIGITAL-SPA-CONTROLLER-WIRE-DIAGRAM.pdf

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check bearings and amp load on motor under load

Motor itself is the culprit.

Capacitor helps the motor to start spinning from a dead stop, so NO, it is not a capacitor problem.

Probably high speed windings. Means - New Motor needed.

Another possibility, is the controller could be sending power to "Both" sides of the windings: High speed & Low speed at the same time. But normally when this happens, you hear a loud buzz type of sound, motor will spin slowly - if at all.

Good Luck

I'm going on somewhat limited information here not knowing if the tub is newly installed or if this is a spa that has been working in the past and just started doing this. It would be helpful to know if this is a recently installed pump as well. That said here are a few things to check.
If it's a new install: Your electrician has more than likely wired the spa incorrectly. Be sure that the spa pack has 220V going into the pack and that the GFCI is wired correctly.
If it's an old spa with a new problem: A lot of times a bad heater element can cause a spa to trip the breaker. If you have a bad element the spa will operate normally for about a minute when you turn the power on, then trip the breaker as soon as the heater kicks in.
If it only trips the breaker when the pump is turned on to high speed, more than likely there is a problem with the wiring of the pump or the relay on the pump itself.

As I'm sure you know the low speed is the filtration & heating cycle.
If you've checked the terminals on the board and on the pump & there's nothing wrong. Use a voltmeter to test if each leg is getting the right voltage. If not its likely the motor is bad.

these are a bear to work on.. there are start switches inside of the motor, and will break if you even look at them wrong.(fragile) you can try compressed air to blow out the trash that might be in the switch..or unhook power cord of the motor from the motherboard and remove the pump and motor from the spa , bring it to your local pool pump moor repair guy with free testing have it tested. if removing wires from motor YOU MUST REINSTALL THEM EXACTLY AS THEY CAME OFF ! OR YOU WILL BURN UP THE expensive 2 speed motor .. jay Florida

the problem with the sound is the bearings inside the motor are going out. i have the same pump and i have to replace the motor. replacing the bearings is very hard and will cost you about the same after you carry the motor and new bearings to a shop to be pressed in. hope this helps!!