Hayward heater cycles in & out of Lo mode starting at 96F

[pwa2003]

Hayward H400FDN heater is giving us this problem: When heating the spa, set to 100F, the heater runs as expected until water temp reaches 96 or 97. Heater then stops, shows "Lo" on the display, stays off for a couple of minutes, then turns back on. This cycle (on/off) continues every degree from that point on until the desired temperature is reached. This all started the day after we had a thunderstorm that caused a brief power outage. Not sure if that's important, but seems too coincidental to not mention it. Is there some sensor we should be looking at?

We don't have this problem when heating the pool to the max set point of 90F.

We understand that "Lo" can often mean it's time to clean the filters, so we did that, and still get the same exact problem.

 

[ajw22]

You can jumper the sensors one at a time, as a test, and see which one keeps the heater running.

A Hayward LO code is a limit string open error that occurs when any of the safety circuits on your Hayward pool heater opens. This forces the heater to shut down to protect your heater from damage.

Usual reason for the LO code is lack of sufficient flow not closing the Water Pressure Switch. Low water pressure can be due to a filter that needs to be cleaned, low pump RPM if a VS pump, open heater bypass valve, low skimmer water level, or even a stuck weir door.

Also on the Limit String is the Temperature Limit Switch, Exhaust Gas Temperature Limit Switch, and Vent Pressure Switch.

See pages 40-43 in the Universal H-Series Diagnostics Guide.