E90 belongs to a group of 5 sub-error codes, i.e. it is the error displayed to you as the end user. To narrow it down you would have to access the service program.
It could be any error from E91 to E95 & will be one of the following: communication error between main PCB & panel card, faulty wiring, faulty PCB, faulty display card, or incorrect configuration.
If you have recently replaced the PCB it may be necessary to reconfigure the electronics, this is model dependent & need not be the case in your instance. If you have NOT recently replaced the PCB then the fault will one of the first 4 above.
Mike, October 2009
