Thermador p304cs/04 error code 21

[Jo jo]

I have a thermador p304cs/04 gas range/oven flashing error code 21, I have replaced the potentiometer, and traced the wiring to the smaller control board and replaced that as well  but the light still flashes 21. I have tried a few other things as well, replaced the ignitor, and thermostat as well hoping that will fix my oven not turning on. But that didn't help. 

There isn't anything online that helps with this problem, so reaching out to see if somebody can help with this, 

Thank you..