A Craftsman furnace has a diagnostic electronic device. It is blinking three times. The igniter is glowing, and then the natural gass starts with a smal flame. Then the igniter goes out and the gass stops. It does this three times and then I get three blinks. Any help?

Sounds like flame senser needs cleaned,, is there a diagnostic sheet glued to the inside of door/cover? Should tell you what the three blinks mean
I`ve never heard that there was a furnace with the name craftsman, I`m guessing it was built by a more known furnace mfg. and has a craftsman label on it