I have a unit here showing E21. There is an IC with a hole blown in the top of it in the heater wire detection circuit. I cannot find any info at all on the IC in question, so a new board stack is required to cure it.
the components on the mr850 cct boards cannot be replaced because the boards are set up at the factory( this is what f & p told me) therefore it looks like you'll have to replace the board.
Just had another unit in here with E21. Replaced the IC in the heater wire detection/monitor circuit from the earlier mentioned unit (now scrapped) & all is good. The IC type will be either 90H948 or 90F009.
We have seen quite a few 850's with this fault, usually around 5-6 years old. We have tried to find the components and failed so we just buy the boards from F&P as well.