I'm guessing it is either a cat or Mitsubishi
If so those were notorious for having that issue
There is a laptop program and cable needed if indeed that is one of those.if it is they had issues with mixer assembly and the rpm sensor to set the mixer you really need a laptop to watch the percentage as you adj
The adjustment screw is hidden behind a small
Plug it is there to discourage adj by untrained
Persons you can guess at it but really need the cable and program does it show a blinking light on the dash ?