Flames goon for 5 sec. then off

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I serviced a Lennox furnace, the thermostat is set for 72 degrees upstairs but the actual temp is only 64, it never satisfies the temperature . The furnace kicks on then the flames go on for about 5 seconds then go out. It it will repeat this a few times then the furnace will just stay off for a while. Then it is back to the cycle.
I cleaned the sensor, even replaced with a new one. Same story. I changed the circuit board, it ran for about a week, same problem returns.
Any suggestions? Do I need to replace the gas valve?

Since you have replaced the flame sensor and the circuit board your problem should be elsewhere. Did you also replace the wire on the flame sensor? That item has been mentioned in other posts as a possible problem also. If you have not replaced it I would consider that next since the wire cannot cost that much. Now if the flame sensor came with a new wire like some do we will have to take another approach.

Post back and let me know if a new wire came with the flame sensor or after you replace the wire to see if further work is necessary.

Flame sensor is new with wire. Resolution to this problem - I replaced the pressure switch and cleaned the rubber tube. Everything is fine now.

Glad you got it going.