Water Heaters - burner wont stay lit

I have a RUUD Pacemaker PH2-40-40F. 40 gal gas vent. I recently had furnace serviced but did not shut off gas or mess with flue. That night and early next morning hot water was fine. In the evening no hot water. Sealed burner. Relit pilot and as soon as dial was turned up burner kicked on for 10 sec and quit. Would not relight. Opened panel inspected and cleaned. Resealed and relit. Same problem. Replaced thermocoupler. Same problem. Replaced gas valve. Same problem. Every time panel is opened it works for 10 sec. As soon as air is used up it will not relight. I have opened panel and not resealed and lit and it works fine until reaching temp. Then shuts down including pilot. I only used this method temp under supervision of my father (a master plumber). Starved for air somewhere. Lit a flame in flue and it all works ok. Arrestor plate seems sealed in bottom. Any help?

It is only 4 yrs old. Was working fine with no problems.

Sounds like it's a combustion air problem. Clean the arrestor plate located under the burner. Also check for any other area the air must enter to get to the arrestor plate. The arrestor plate has many very small slots that are easily plugged with dust and lint. Very common problem. If you have an air compressor use it. If not use a brush.

Seems the arrestor plate is covered by a filter/ grate that is riveted to the bottom inside burner chamber. I can't access directly. I used a shop vac to clean out inside area I could access and outer air intake holes. The arrestor plate is on some type of sliding guide that moves freely inside. Any special way to get inside?

I have a rheem water heater and it is doing the same thing. The burner will only stay lit for 5-8 seconds and then go out. I have replaced the thermal couple only.