I found a burned resistor and an open resistor. Several others are hard to
read.

I need values on the following parts:

R804 - burned
R806 - questionable
R421 - questionable
D851 - Bad if it's a diode, but I can't figure out what it really is or how
to test it.
Clues:
Two leads coming from a small rectangular plastic box.
(It gives the same reading with an analog VOM on Rx1 no
matter which way the leads are connected.)