Kenh

Yeah could be if he didn't know 

24 minutes ago, klm said:

Yes it does, hard to change.

How old is it, how many hours/miles?

I have a 2017 Massimo MSU500 side by side. It's not really that hard I have changed mine a couple of times to include the clutches primary and secondary also the wet clutch.

I had the same problem with my 2017 Massimo MSU500 side by side.

I replaced the ecu it controls everything 

To my guess form what you're saying it sounds like the riv limiter. That is protecting the motor. I have a 2017 Massimo MSU500 and when I replaced the ECU you have a procedures that you use to reset it and you increase the rpm to set it which is about 3 to 4 hundred rpm

Massimo MSU500 won't shift to low

I would check the shift linkage. Adjust it make it a little longer, if it is not long enough it won't shift to low 

On the massimo msu500 , Start the motor, then press on the gas all the way and let the riv limiter cycle holding the gas pedal all the way down, turn off the ignition, then let up on the gas pedal. Then it should be reset.

It could possibly be in the fuse box. Mine is under the hood fuse and relays

I had to replace mine