Laars 400LT Natural Gas Spa Heater Shutting Off Too Soon


Well-known member
Nov 29, 2011
Magnolia Texas
I've got an in ground pool and spa that is nine years old. Both are heated by a Laars 400LT natural gas heater. Yesterday, when I tried to heat the spa for the first time since last winter it fired up fine, ran about 40 minutes until the water reached about 90-92F then shut off. There were no warning lights on the control panel at all. I waited 10 minutes, turned off the entire system (pump too) and restarted it. The spa heated for about 5 minutes and shut off again. If I tried restarting to quickly nothing would happen. By waiting, restarting, having it shut down and repeating I eventually got the water up to 98 degrees after another hour. My system is set for 102F through my Aqualogic P4 control box.

Since the system fires up fine and heats until the water hits around 92F I'm thinking a sensor such as the exhaust, high temp 135 or 150F might be the issue but before I start throwing parts at it I thought I'd ask the experts. I already checked the troubleshooting guide and I couldn't find anything helpful in there. Anyone have any ideas? Thanks in advance!


TFP Expert
In The Industry
Jul 6, 2011
Yes sounds like the high temp sensor might be the culprit. You can test it by removing the sensor and put a jumper wire in its place. If the heater kicks on, then you probably found it. Obviously wait till the unit gets hot enough to shut off and then put your jumper in place.