AS you found out, when you get a NOCO, if you fail to find and correct the reason, it locks you out. Now, at this point we need more info.
1. Is the refer in a slide room?
2. Have you verified that the upper roof vent is clear and open if not in a slide room?
3. 3 things usually cause NO CO
1. Blocked/inadequate ventilation
2. BAD cooling unit
3. Running off level
4. At this point as long as ventilation checks out OK, you connect the refer 120 element direct to 120 volts and run 24 hours. If the refer does not get the freezer below 10 degrees, the Cooling Unit is bad. Doug