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Jandy Spa Heater LT400N-L cycles on/off


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I have a Jandy Pool/Spa Heater, model LT400N-L (made 6-2006) and it stopped working. Red light on Pressure Switch. I put it on the to-do list for about 2 years. Recently, replaced the pressure switch and now it will fire up, but it cycles in what seems like high/low moments for 3x and then shuts off (safety loop). Water is flowing, fan comes on, I see the glow in the tiny peep hole. Fan comes on about 35 seconds, then a cycle of warm air comes out and fan gets louder for 15 seconds, then warm air ends and fan seems to drop a speed. During the warm air/ high fan cycles, the water does pump in hot into the spa. I have the Jandy PDA and the AquaLink RS cabinet–water temp registers spa=74 on the PDA and I have the temp set to reach target 102. The high limit switches are in excellent condition visibly but I still bypassed them with a jumper, no change. I found the diagnostic flow chart online, tested all the voltages, all was well.  I did notice that the brown wire on the gas valve only received 24VAC for a second or two, so maybe the gas valve is not being allowed to remain open and flow, but why?  Perhaps this contributes to the problem?  Could it be the ICM? Before I gamble on such an expensive part, is there a way to bypass/jump/test this brown wire terminal, or do you see something else I could check?  I’ve also replaced the flame sensing rod for fun, no change.  I was thinking about replacing the temperature sensor but would prefer to diagnose logically than keep guessing with parts.  Appreciate any feedback, input or suggestions.

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I am no pool heater expert and have never seen this one, but I have never seen a 2 speed blower in any heater. Check voltage to the blower. Check the vent pressure switch and verify it is working properly. 

Can you see the burners? If so, is it burning blue or yellow? 

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Update, so I went to check the voltage readings on the blower and pressure switch and noticed a new error, AGS.  Upon closer inspection of the NEW flame sensing rod, i found it to be significantly shorter than the old one--so no way it could reach the flame.  I put the old one back in but the set screw which was heavily rusted out I replaced with a new one.  I turned it on and it worked!  We enjoyed the spa for a good 45min last night with no issues.  Hard to believe a rusty set screw, which I know can impact the ground of a circuit, was the issue!  I plan to test again tonight and confirm it remains working.  If so, we'll consider this case closed.  Stay tuned...thanks to RDspaguy for the reply and giving me the incentive to go out and look at it again!

flame.jpg

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