Jandy Laars LT Boot Up Symptoms - Which part most likely needed to repair?

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Jan 4, 2014
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Been 10 years since my last question so I'm way overdue.
Unit: Jandy LAARS LT Pool Heater
History: Past few years I've replaced a Pressure Switch and a Run Capacitor. In 2017 I replaced the 150 deg Hi Limit Switch. Not sure why I didn't also replace the 135 deg at that same time. Stupid.
New Symptom: Turning on the Spa mode, all seems to start up fine with the pool to spa dual valves turning fine into position. After the fan runs high speed to clear out the air in the heater, I hear the gas line valve click on, the burner fires up, the fan is on a lower speed and HOT air coming out of the (clean) vent. A few seconds later when fan switches to high speed, the burner suddenly shuts down. It then recycles all over again trying to fire up successfully. Never does.
What I'm thinking: I don't want to be shot gunning parts that are not the issue. Could it be the other Hi-Limit switch (135 deg) that was never replaced OR be the Temperature Sensor? Or some other part?
Thanks for any suggestions others may have!!
 
I have the LX version of that heater and mine has a single speed vent fan. I would be surprised if the LT version doesn’t have the same vent fan.

Have you checked the fan pressure switch to see if it is opening up (and shutting down the flame) with the slow fan speed you are describing?
 
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Thank guys for that input, especially the block diagram PDF! After I wrote that first post, I thought about the fan pressure switch as something also to check. I'll take another look today and keep all informed. Fortunately heaters have few parts that fail and usually simple to repair. Thanks again.
 
I finally had time to work on this heater today. For sake of ease, the first thing I did was the suggestion from ajw22 and removed the Flame Proving Rod and although did not appear too discolored, I used fine steel wool to clean the rod. I then started the heater and with disbelief, the heater stayed on. How could it be this easy of a fix? We'll see what holds in future starts but it's running now and spa time coming up shortly! Oh, I might had the sequence fan speed sequence incorrect in the earlier post.
Sequence upon Spa start: fan runs at slow speed, gas value opens, burner ignites up okay, feel hot air from vent while fan still at slow speed, then fan kicks on at high speed, burner stays on (earlier the burner shut down at same time the fan kicked into high speed.
Keep all informed if anything change. THANKS TO ALL for your response and great to have the troubleshoot block diagram.
 
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