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Laars Endurance F4 Error Restarts Periodically

jeffceojeffceo Member Posts: 18
Hi Everyone, This is a combination propane boiler for heat with domestic hot water as well. While the unit is running, sometimes the burner will suddenly shut down and an F4 error code will appear on the screen. It will then wait a minute or so and start the burner again. F4 is a supply sensor error. The sensor in in the bottom of the boiler.

I thought it could be overheating and causing the shut down but the screen shows around 180 and I get around 160 on the sensor nipple. Also, the internal circulator pump does seem to be running as it is drawing .70 amps and I can hear the motor if I put a screwdriver to it and my ear. I made a video of what happens here:

There have been a handful of times where we had cold water coming out of the faucet instead of hot but the hot kicked in after a minute or so. Not sure if that is related or not. Let me know if you have any ideas. Thanks!


  • unclejohnunclejohn Member Posts: 1,527
    My idea is too replace the supply sensor.
  • jeffceojeffceo Member Posts: 18
    Hi Guys, I replaced the supply sensor (copper and brass sensor on the bottom nipple of the boiler combustion chamber) and it is a 100% improvement. The sensor should be 10K ohms at 77 degrees. I was getting just over 1k.
  • jeffceojeffceo Member Posts: 18
    Sensor part number is 2400-445
  • BorNYBorNY Member Posts: 1
    Hi Everyone, I have the same exact problem described in this post by Jeffco. I am not a plumber or a handyman but consider myself handy. I ordered Supply Sensor part number: 2400-445 and was about to replace it myself, but I could not get to the part on the boiler since it is in the back of it and not possible to get to it by hand to unscrew the sensor from the bottom of the boiler combustion chamber.
    My question is, in order to replace this part should the right side of the boiler's cover need to be taken off by unscrewing all the side screws so I can get to part or is there another way which I cannot figure out which requires special tools.
    Please let me know.
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