Have a Hayward H400 FDP. CE error, probably year or so. If the boards have to be replaced, why together, versus replace the one that is bad?

ensure your getting 24volts to display circ board. you will probably need to replace the display screen

Communication error

If communication between the ignition board and the display board is not established

within 3 s of power-up an error will be displayed. After communication is

established, if it is lost for 30 s, the error will be displayed. The error code will
be cleared upon a valid data exchange between boards.

It means Communication Error. The display board and the Ignition board are not communicating with each other. Look for rodent damage first. If not that it could be either board and the only way to determine which one is by replacing one and see if it fixes the issue. Due to he cost of the boards it usually pays for itself to call a Hayward certified repairman out as he/she will have both boards available to try ond only replace the one you need.

I have just repaired my H400 series pool heater from the 3rd CE error in only 3 years since the pool and heater were installed. In each case the circuit board (F0059-456600) was in some way to blame .... I will explain.
Case 1 : Only months after installation the CE error came up. Rain water had got in around the display module and water-damaged the circuit board. It was replaced under warranty. Since then I keep a big plastic lid covering the display unit when the heater is not in use.
Case 2 : 1 year from new the CE error came back. On investigation it was found that a rat had managed to get inside the heater (there are big gaps around the pipe entries) - it had then partly chewed through the ribbon cable joining the two circuit boards together. Display circuit board was replaced at my cost - and all was well again. I also rat-proofed the heater - eliminating all the big openings with chicken wire.
Case 3 : Fired up the pool heater for the first time this year and CE came up. I pulled the display module and this time found obvious signs of blackening / burning on the display circuit board under the big green resistor (R1). Luckily I had kept the old circuit board from the previous time and was able to make 1 good board from the two I had - back in action now.
So in my experience it has been the display circuit board each time - and it only takes a phillips-head screwdriver and about 10 minutes to get it out.

CE is a communication error between the display board and the ignition board. If you've had a lot of rain lately it could just be that moisture inside the unit is causing the error rather than a bad board. Take the cover off and blast the inside with a hairdryer for a few minutes, making sure to get the boards and connecting wires. That sometimes clears the error.

The P at the end stans for propane...the manual found here is for the H400FD series natural gas or propane.

John

Code "CE" stands for communication error and describes a fault between the mother board and the display. I doubt it's the relay causing it though...

CE is a communications error on the board...the control module and/or Display interface assy. probably need to be replaced.