Good idea from Smokey. Trouble shooting is a process of elimination. Seems the most obvious causes have already been checked. Although voltages should probably also be a priority. Definitely check the voltage to the combustion fan.
You could also remove the 4 screws on the combustion cover and pull apart the two snap together wire connectors at the combustion blower . Directly feed 120 volts from a reliable source (check it) and connect it to the comb fan. It will run at full speed but will only be running slightly faster than on the lowest speed anyway so it won`t matter.
Start the stove and let it run for an hour , then check it for a clean burn. You shouldn`t have any indication of black smoke exiting the vent and the flame should be bright yellow and quite active with little to no dark orange flames present.
You could also remove the 4 screws on the combustion cover and pull apart the two snap together wire connectors at the combustion blower . Directly feed 120 volts from a reliable source (check it) and connect it to the comb fan. It will run at full speed but will only be running slightly faster than on the lowest speed anyway so it won`t matter.
Start the stove and let it run for an hour , then check it for a clean burn. You shouldn`t have any indication of black smoke exiting the vent and the flame should be bright yellow and quite active with little to no dark orange flames present.