I have a carrier 58RAV induced combustion furnace. It has been blowing cold air for a while now, and I am trying to fix it. The fan turns on but the flame never gets lit. The LED status light is blinking rapidly with no pause. According to the user manual this means... "Check for correct line voltage polarity. If units are twinned check for proper low-voltage (24V) transformer phasing" I do not know what that means. How is it possible for the polarity to change?

Thanks!
Eddie

Usually polarity does not change.
LOOK for a loose ground wire at the main electric panel through the furnace on/off switch and to where it is grounded in the furnace itself.

A ground might look good but is not good enough so it has a effect on the computer circuit board in the furnace. In other words just do not look for the grounds but make sure they are clean and tight.

Are your units twinned. In other words are there two furnaces set side by side and connected together ?