Hey how are you
I will assume it is a diesel
cause these codes are diesel specific
P0284 ; # 8 cylinder contribution/Balance
P0278 # 6 cylinder contribution/ balance
P0308 : # 8 cylinder misfire ( most possible cause fuel injector or wiring to it )
I have sent you the diagrams & possible causes for these 3 codes
it looks like the injectors are not firing or clogged
most likely the p0308 is caused by the p0284 , if the injector not firing on # 8 it will also show misfire .
so if you fix the p0284 , the p0308 might go away after that
do a wiggle test on the harness on the injectors
with a proper scanner you can test the injectors
Please let me know if you need anything else on it, just reply back and we will continue
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ASE Certified Technician