won't heat

My question in I’ve had where I put a machine in diagonistic mode and the engineer that design the computer board didn’t flag the right part to replace with the code that came up. This makes are job harder. How to you know for sure when a wash machine has a computer board and the interface board which is your digital display, which tell you how many minutes the wash cycle is a time dry and the codes displayed. I had one time that it coded that it was the computer board I replaced it and it wasn’t the problem. Whirlpool I called they told me replace the interface board. I did that and that wasn’t the problem. These two boards cost almost $450.00 On this particular whirlpool hand another board that communicated to the computer. That one was replaced also. still didn’t fix the problem. All the board were checked for power and all output relays were checked to see if the output where working or not. All I want to know is there a better way to check the board because diagnotics isn’t always right what the engineer says what it is. If you could imagine this machine to try to fix would of cost over $600 dollars plus labor charges. I called whirlpool and they told me they didn’t know why it wasn’t working. funny thing though it work in diagnotic service mode but you put it back for the customer it wouldn’t work. Very strange.