My 12 yr old concord 90 plus when it shuts down after temp is reached ,with the blower still blowing will re-ignite,run for 30 secs then shut off, re- ignite again for 30 secs again and does this about 6 times before it finally shuts down. Then it will not come back on after temp drops, then I get 6 flashes on the gas valve, I shut off the gas valve, shut off the gas, then switch of the power wait 15 minutes then turn on the gas.valve and the power then it starts and runs for 1, 2 maybe 3 hours then goes through then process again, What might cause this to happen, It ran great when I turn the heat on in the fall,until it gets cold then it starts to act up again. sometimes it starts the same way,ignites for 30secs then shuts off , ignites again 4or 5 times, then it shuts down and goes into lockout.

Make sure your intake and exhaust vent are not restricted, I had a wasp nest in mine. Also polish the flame sensor with steel wool or 400 grit wet or dry.