Norcold N641 not cooling on propane

1. IF the refer was operating until the 120 went out, I would install the new 120 element and wire it direct to 120 for 24 hours
2. The NO CO will NOT happen if the refer is operating to spec and the LP or 120 fails and the refer warms up. It will only throw the NO CO code when you start it up and it CANNOT do a correct cool down from ambient inside temp.
3. You get 2 tries at NO CO. The FIRST lets you know there is a problem and if you do NOT correct the problem and restart the refer, it throws the 2nd CO CO and locks out the controls and requires a HARD RESET. Since you stated NO CO and did the jumper reset, you had a previous 1st time code. Since you did not fix the problem you had to do the jumper reset.
4. As Biscuit, stated, these are the things to look at, most common is rear cooling fans if you have them.
5. Is this refer in a slide room?
6. GAS pressure is critical. Low pressure will still yield what some will think is a good flame. Have you EVER operated the refer on LP since you replaced the CU? If not, if you did not use the flue baffle(Amish cooling units may not come with them, you swap from old CU), then the refer will not function on LP. Are you positive the unit has the Spiral Baffle in the flue tube? It is not required for 120 operation. Doug

'No Co' operating on GAS
Level
Ventilation obstructed vents (lower/upper) birds nest/fallen insulation
Flue cap obstructed
Gas Pressure
Cooling Fan not running (IF equipped 'F' models)

See Pgs 39/40 to test Cooling unit capability

N6XX/N8XX Service Manual