Laars Jandy LT 250 LoNOX

[millerwd]

Laars LT 250 pool/spa heater will ignite and burn for a few (3-5) seconds , then shuts off. Blower will start, run for about 1 minute while igniter glows, the gas valve clicks open, gas whistles through nozzles, the burners ignite (visible through the site glass) then after a few seconds the gas supply shuts off. This occurs with either pool or spa mode and the pump running at high-speed.

After three ignition cycles the diagnostic code AGS (Automatic Gas Shut-downy) displays. I installed a new flame sensor, but no change. I've vacuumed and blown out the burners, checked each nozzle for blockage, verified the flat surface igniter is working (glows orange through the site glass), and verified good water flow. I have the manual, but am not sure what to check next. The unit is 13 years old, has a bit of rust, but has been well maintained. I replaced the main control unit about 6 years ago (lightning strike).

 

[MrApplianceMatt]

What is the amp draw on the igniter? Typically the safety valve is looking for an amperage threshold to be reached. Gas ovens require igniter to draw 3.2-3.8 amps to maintain the safety valve open. A weak igniter (3-ish amps or lower) will glow and may even start but cause heating issues as the current struggles to maintain the safety valve open.

Edited June 12, 2019 by MrApplianceMatt

[Budget Appliance Repair]

@MrApplianceMatt This doesn't sound like it is like a range - more like a forced-air furnace which doesn't rely on amperage of ignitor.

Sounds like a flame sensor, (which has already been replaced with a new one, not just cleaned), so it would be a ground issue or main board problem.

I don't know this system nor did I look it up but from the description this pool/spa heater sounds like it operates on the principles of a forced air furnace and the flame is actually igniting then seems to be dropping out in the 5 - 6 second flame proving period.