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This can also be caused by a clogged vent line for the toilet. You would probably need a plumber for this.

In addition to Tom's answer some toilets syphon jet is under the lip were the water goes down the toilet. You have to put your hand in the water with your fingers under the lip and flush the toilet....

My furnace will run fine for a while then go into lockout (2 flashes). It goes into lockout when it tries to start up normally. It does everything normally except when I here the gas valve click it...