"LO" error code on new H400FDN heater

this indicates a communication error between the heater and control module. it may be as simple as disconnecting the main power to both the heater and control module to reset system. be sure to allow enough time for the system to 'forget'. usually 1 minute is fine.

make sure wiring is correct especially if wired 120 volt read instructions inside panel

"LO" is simply informing you that the Heater is not sensing proper water flow through it! A Flow Problem.
A special sensor, called a pressure switch is this source. But... That does not mean it is bad - No, it is in most cases just doing its job!
Most common reason - Dirty Filter.
A good cleaning of the cartridges, or maybe a breakdown and recharge of a DE Filter, or a good cleansing backwash of your sand filter, will remedy this situation.
Or... Check for any OTHER reason or situations water flow may be reduced through the heater:

• enough water in pool or spa?
• If you have a weir gate in skimmer, is it stuck in the UP position?
• Are any valves closed?
• Do you have a bypass valve at the heater?
  • If open, this will reduce the amount of water going thru heater, close it to the point, LO goes away.
• Could be debris in front of Pressure Switch, in the copper tubing
  • Unscrew and remove pressure switch, turn pump on. But first, align end of tube so that when water squirts out it will shoot out from heater and not onto electronic heater components!
• If all above is good, you may have failed pressure switch?
• Etc

Good Luck - hope this helped.

"LO" is simply informing you that the Heater is not sensing proper water flow through it! A Flow Problem.
A special sensor, called a pressure switch is this source. But... That does not mean it is bad - No, it is in most cases just doing its job!
Most common reason - Dirty Filter.
A good cleaning of the cartridges, or maybe a breakdown and recharge of a DE Filter, or a good cleansing backwash of your sand filter, will remedy this situation.
Or... Check for any OTHER reason or situations water flow may be reduced through the heater:

• enough water in pool or spa?
• If you have a weir gate in skimmer, is it stuck in the UP position?
• Are any valves closed?
• Do you have a bypass valve at the heater?
  • If open, this will reduce the amount of water going thru heater, close it to the point, LO goes away.
• Could be debris in front of Pressure Switch, in the copper tubing
  • Unscrew and remove pressure switch, turn pump on. But first, align end of tube so that when water squirts out it will shoot out from heater and not onto electronic heater components!
• If all above is good, you may have failed pressure switch?
• Etc

Good Luck - hope this helped.

Not at all uncommon. New heaters have to glaze over the file tile in the burner chamber and will often smoke and smell during the first week or so of operation.

Using a mirror, check to see that your flames are about 4" proud of the burner tubes when running. As long as they are and the color is a nice blue with little to no yellow/orange color, the heater is fine.

Is the heater installed indoors?? if so you'll need a vent pressure orifice kit.
if not check your water press. (below 18 PSI on filter) Place filter on "recir" if it fires and stays on . it's the pressure.
check to see if all connections a set tight hi-ltd switch wire harness etc.

I had the same issue...you don't have enough gas pressure.