Concord RGH80 will not heat. Blower turns on. Diagnostic lights (both) flash constantly. No flames. What can cause this and what part or parts are needed to repair?

OK look on you furnace door or you owners manual to see what the flash codes mean.

Is your furnace vented with metal vent or plastic?

Watch the blinking lights closely. See exactly what they are doing, then do as josh said and find the diagnostic code on the inside cover of furnace, or in the instruction(install) manual. These lights are giving you a code that tells what is happening to the furnace. If you can let us know what the code says, we may be able to help you fix this.