My Hayward 400 has no lights on the panel, no codes or modes

This is a normal display when heater is being controlled by a remote thermostat. No service is required. If heater is not being controlled by remote thermostat, change setting by using the MODE key to put the heater into STANDBY. Press and hold the DOWN key and then press and hold the MODE key. HOLD down both keys for 3 seconds until the indication "bo" is removed from the display.
The key to this is DOWN first and then together with MODE, both for 3 seconds.

Make sure the display is not showing "BD".

Hi
If you have a auto control system like a Aqualink,Aqualogic Put the controller in service than select heat. Check and make sure the service light is not on also. What happens when you press the mode button. If nothing and the button membrain looks good. open the front panel there is a ribbon cable that goes to the display and plugs into a control board in the back. Unplug the cable and plug it back in "Be Careful The Pins bend very easyly. Try the mode button again. Is there any error codes on the display?

MAKE SURE THE FILTER IS CLEAN THAN THE TEMPETURE SENSOR MAY BE BAD, CLEAN THE FILTER FIRST

OK I GUESS U HAVE A AUTHOMATIC CONTROL CHECK T RELAY FOR 120 V THAN CHECK FOR V AT THE JUCTION BOX IF V LIGHT MUST BE REPLACE.

There is a thermistor temperature sensor beside the inlet union on the heater that goes bad, it looks line a stainless steel nut with wires on it. You can unthread to replace it and it plugs into the board behind the display. The LO means low pressure probably caused by pressure switch adjustment.

check millivolt output,if it is in normal range(400-600)check for continuity on pressure switch,flame rollout and high limits.