http://www.graycoolingman.com/uploads/1/0/6/6/10667336/58mvp-_2_stage_variable_speed_troubleshoot_guide.pdf
Open the troubleshooting guide and go to page 13 for Fault code 12. You can scroll from there to find the fault code and the troubleshooting steps recommended for fixing the issue.
You mention you used test mode. I believe this makes the system go through the stages of operation and then turns off. If you checked it only for a short period with the blower door off during self test mode, I'd put a jumper in, described in troubleshooting, and cycle the system. The jumper bypasses the thermostat and will allow the furnace to run for an extended period. I'd make sure you're in the high fire mode (I think you can jumper to high fire) and watch it with the door off to see if the code is set.