I have a Classic 90 Series Rheem RGRA series upflow/downflow furnance 3 years old. Intermittently, the furnance will not flame up or give out heat. Sometimes it only blows cold air. Sometimes, it locks out (middle green light flashing once then pausing). The furnace repair company came out and said it was the gas line pressure blowing out the flame. That has been ruled out by the gas company, who checked the inflow and outflow pressure. Are there any suggestions? The furnace is still under warranty.

Well, the service company should be replaced by someone who has a clue. Rheen/Ruud has had more than a few problems with faulty pressure switch's (5 year warranty) I don't know off the top of my head what 1 flash means, it is on the blower door tho, also, if the furnace is not properly draining the water, it will act like a pressure switch issue. Good luck.

I have a Classic 90 Series Rheem RGRA series upflow/downflow furnance 3 years old. Intermittently, the furnance will not flame up or give out heat. Sometimes it only blows cold air. Sometimes, it locks out (middle green light flashing once then pausing). The furnace repair company came out and said it was the gas line pressure blowing out the flame. That has been ruled out by the gas company, who checked the inflow and outflow pressure. Are there any suggestions? The furnace is still under warranty.
We have same furnace and have had the same problem. Also ignition problem and now the furnace clanks/bangs just before fan comes on.

Well, the service company should be replaced by someone who has a clue. Rheen/Ruud has had more than a few problems with faulty pressure switch's (5 year warranty) I don't know off the top of my head what 1 flash means, it is on the blower door tho, also, if the furnace is not properly draining the water, it will act like a pressure switch issue. Good luck.
I agree with NorthernHeat check the drain lines on the furnace. Behind Top door two rubber lines going to drain trap left side of furnace. Trap will exit furnace behind lower door of furnace(blower door). check for a kink in the line behind top door.(rubber line 1/2 inch)

I have a Classic 90 Series Rheem RGRA series upflow/downflow furnance 3 years old. Intermittently, the furnance will not flame up or give out heat. Sometimes it only blows cold air. Sometimes, it locks out (middle green light flashing once then pausing). The furnace repair company came out and said it was the gas line pressure blowing out the flame. That has been ruled out by the gas company, who checked the inflow and outflow pressure. Are there any suggestions? The furnace is still under warranty.
new ignitor

new ignitor
If furnace is only 3 years old it has a spark ignitor, not hot surface and shouldn't need replaced. Installers often leave to many pvc filings in the pipes plugging the drain in the venting transition tee or in the trap, it seems to take 2 or 3 years to cause any problems.