I have a Armstrong ultra V tech 80. G1N80AU075D12A-2. It is about 6 years old. The filter is clean, the flame sensor is clean, and the vacuum line/ fitting is clear. I have gas because other appliances are functioning just fine. I turn up the thermostat and it performs a purge as normal for about 15 seconds. The ignitor then comes on and shortly thereafter the flame (gas) comes on and it lights momentarily and goes out (only about 1 second of flame) It then goes into lock out with a 6 then 5 flash which tells me the gas (flame) should not have occurred. Please help, and thank you