Error codes, chicken or egg

Model Number: WDT790SLYM3
Brand: Whirlpool
Age: 1-5 years

This dishwasher has been giving me problems the past year and I have corrected a couple of problems that were indicated by error codes.
First was 7-1 heater, so I replaced the heater element and that went away, however the next code to show up was 6-6 OID/temp. probe reporting
water temp low but wasn't so replaced OID so that went away. Now I have 8-1 slow drain and wondering if a bad/restricted water valve could cause
this or a restriction in drain hose or bad drain pump/impeller? Troubleshooting chart also says Check Valve in drain loop but not sure where that might
be located.
Any help/comments/suggestions welcome.
Thanks

OK, so I tried testing the water inlet valve as shown on the video here on testing this valve. Water flowed OK no issues.
So now I am thinking the soap dispenser may not be opening at the right time. It could be opening at the end of the clean cycle
before heat to dry dishes. I say this because last time I tried washing dishes you could see where the soap had drained out
over a plate and to the bottom of the dishwasher so looked like no water was being used however this will also look this way
if the soap dispenser doesn't open in time. So how to test the soap dispenser? No error codes for this possible problem?

So no help with this error code and from what I see on this forum not many are getting answers to their questions :-( . So will try again in different thread.