That sir is not possible. 120 volt AC in North America is a live leg and a neutral leg. Single phase power is 230 V that is phase to phase. The Stator will have 2 sets of windings. They tap off the middle of the sets to make a neutral. Neutral will be bonded to a earth ground at the source. Unless this is one of the new inverter models. They do not care about engine speed for cycle timing but instead make DC power and electronic switchable diodes to produce 60 cycle AC from the DC. If this is what you have, check the owners manual for any special requirements they have incorporated for safety.
Oh I forgot to ask the size of it and what plug your working off, brand, And the furnace voltage and current requirements.
Ok I might have mis informed you on this. The Generator has power on both prongs of outlet which is a total of 120 volts. The furnaces control board sees a voltage polarity issue and will not operate the Ignition sequence. This is not for me personally. I believe the generator my friend was using was not and invertor style generator just a cheap portable 120 volt generator which was most likely not grounded . I have studied this and I have not found a solution to this problem. I know its most likely due to not a true sign wave and PWM. Other than that I was not there personally . Was wondering is there a noise filter of or devise that would correct this issue.
Standard push in receptacle. The issue is furnace is throwing polarity fault. It has something to do with either not being true sine wave or PWM or generator not being grounded. I am asking this on be half of a friend I was not on site personally . So everything is second hand knowledge