Heater shuts down after 30 seconds

I have a Laars LX400 propane heater that has worked pretty well since I installed it ~5 years ago. Starting up the pool this year it lit right up and ran fine for maybe 12 hours when I shut it down. I had a few other repairs to make on other systems so I just got back to looking at the heater. I turned it back on with good pressure on the pump and it starts then shuts down after maybe 30 seconds. It goes through it's start procedure a few times, each time lighting, running for maybe 30 seconds, then shutting down. After three tries, it shuts off and reports an AGS (Automatic Gas Shutdown) fault.

I verified I have plenty of propane - the pump is running great at about 15psi, and I can see flames on the burners when it is running. There does seem to be too much yellow in the flame - but its hard to know.

A few years ago I had heater problems and had the propane company come out and verify the manifold pressure and pipe sizes, ventilation and air supply. Nothing has changed.

I'm stumped. The AGS error diagnostic suggests that it can be caused by filter pump oscillation (not happening), incorrect gas pipe size or blockage, or some sort of loose wiring or problem with igniter, flame sensor or gas valve.

Seems like a lot of things that could cause the issue and I'm not sure where to start. Any ideas? Recommendations? I'm afraid there are no qualified pool techs anywhere near here, nor any to be bribed into making the trip... I've tried.

Thanks for your help!

You say the flame looks too yellow. This suggests that there is debris in the burner tubes, so you are not getting a good mix of air and gas. Use a flashlight and look into each tube. All should be clean inside, no spiderwebs etc. There should be some sort of adjustment for air/fuel mix, make sure those are set properly.

A pic of the burner assembly (unlit) would help!

Charlie

Update: I dismantled the burner assembly and found that three of the burner tubes were damaged in the center where the slots are. One looked like the internal tube was burned and/or rusted through. I've ordered new tubes and will install and retest. Thanks for the advice, Charlie!

...heather