The symptoms started a few months ago with the Flame roll out switch tripping and setting a code. Upon resetting switch, furnace seemed to work properly for a couple of days. Then gradually over several weeks, the rollout switch needed to be reset hourly. When the burner flame ignites, flame quality seems poor and flame does not appear to be completely entering into combustion chamber, thus combustion chamber does not heat up, resulting in no(little) warm air in ducts. Rollout switch seems to be doing its job as temperature is heating up in ignition area.

Steps taken: Replaced filter, inducer motor works fine, cleaned exterior of combustion chamber, verified proper blower operation, cleaned burner orifices (4). I suspect that the gas valve is faulty in not letting enough gas in to "push" flame into combustion chamber. Possible incorrect line pressure, but other appliances work fine.

Is the gas valve a likely cause or what have I missed?

That is not how it works. The valve doesn't "push" the flame into the exchanger as much as the inducer "pulls" it. If the incoming pressure dropped off you would most likely see one or two burners ignite and the unit would ultimately shut off on the flame sensor. If your roll-out keeps tripping and the flame is not being pulled into the heat exchanger then I would suspect either something with the inducer/flue or a cracked heat exchanger. Either way you need to shut off your furnace and call in a service tech to inspect it.

I agree call for service since this unit should have been looked at by a professional when the problem first surfaced.

What you describe sounds like a restricted secondary heat exchanger (polypropylene lining collapsed). Go to furnaceclaims.com, your unit may qualify for the detailed warranty benefits.

Carrier and bryant have a string of those problems.

Thanks you all for your input! It is indeed a restricted secondary heat exchanger. New furnace will fix it. 15 years old is probably not worth repairing.

Did you check out the website? W A R R A N T Y!! Why would you think a freebie isn't worth it?