My Hayward h400fdn is locked out on code 10 combustion fan comes on for a couple of seconds ?

Water Temperature Sensing Error

A) Inlet water temperature sensor is reporting a temperature in excess of 104 degrees. Normal operation resumes 2 minutes after inlet water temperature sensor reports temperatures of 104 degrees or less. This error functions in both normal and remote thermostat modes.
or

B) Inlet water temperature sensor is reporting a temperature change at a rate faster than 6 degrees in 60 seconds, indicating a potentially damaging low water flow condition. Normal operation resumes when temperatures stabilize. If this condition is detected 3 times, the heater will lock-out until the electrical power is cycled off and back on.

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

Turn off the breaker to the pump for 30 seconds.

It is probably not sensing flame. You either have a clogged orifice underneath the flame sensor and you probably need to have the gas pressure assessed, meter size and gas valve pressure adjustment. These heaters need 2" wc of gas pressure.

Pull your heat tray and clean burners and check for rat nests. A common cause of this is spider webs in the gas orfaces on the supply pipe. If you remove the brass orfaces they may have small,small,small tiny webs inside them oyu can blow out or clear with a toothpic. they dirvet the gas from the main burner (the one with the ignitor) as the gas builds it will poof and start but then the other burners that have gas flow won't be close enough to let the ignitor read flame so it will cycle just like you describe. My kingdom for a toothpic. I have made a tool to do this out of a stripped wire bent just so with the end roughed up to catch the webs so I didn't have to pull the tray on heaters with this proble. You should pull the tray and see the whole problem to understand it better.. I hope this helps and good luck.

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.

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