Model # WFW9600TA01 about 10 years old
On normal wash cycles the washer displays F35 or SUD shortly after the cycle starts. Usually after it starts to fill.
I ran the diagnostics (drain/spin; no spin; steam 4x in 5 sec). The diagnostics cycles perfectly; C0-C9.
If I take the tube off the fill sensor, the tub fills indefinitely. If I unplug the electronics from the sensor it errors with F21. I have blown the tube out.
The steam cycle works and the steam diagnostics says yes (start and steam buttons at the same time).
The CCU seems to be OK by a visual inspection. I don't have the tools to check the capacitors on the board. I am limited to testing it with a multimeter.
I have blown out and inspected the drain hose. It has no obstructions. The tub drains well.
When on drain/spin, rinse/spin, or Clean cycle, the error codes don't show up.
I have ran it through diagnostics and clean cycle many times. There are no suds in the water. The first time I cleaned it after the F35 error the water was a little sudsy. Now the water does not keep foam on top more than air escaping.
Any other tests I can do to verify which part it is? Buy and try is pricey with this model.
Thank you.

Here are some manuals: https://dl.dropboxusercontent.com/u/8010163/Whirlpool%20Duet%20Failure%20Codes.docx
https://dl.dropboxusercontent.com/u/8010163/Whirlpool_Duet%20Job%20Aid.pdf
https://dl.dropboxusercontent.com/u/8010163/Whirlpool_Fault_Codes.docx
Go here for fault codes, too: Appliance Fault Codes - AppliancePartsPros.com (http://www.appliancepartspros.com/repair-help/appliance-fault-codes.html#Whirlpool%20Duet%20Front%20Loading%20Ga s%20and%20Electric%20Dryer%20Fault%20Codes)
What you have is two bad capacitors on a circuit board causing a suds error. This is common due to the use of Taiwan capacitors that go bad. Get Japanese capacitors which you can see have 1/3 the top painted red or blue to indicate that they are Japanese. (See: TweakTown Forums (http://forums.tweaktown.com/processors/37513-intel-s-latest-32nm-clarkdale-platform-core-i5-661-biostar-h55-oc-5g-air.html)You) can replace them very easily for about $5.00 with a good soldering iron and 60/40 solder. See: https://www.youtube.com/watch?v=GqQC6oZvCaw
This was a bad problem in the late 80s after someone stole the formula to make solid capacitors (A big trade secret.) and sold it to the Taiwanese, but unknown to the thief he did not have the entire formula and process. This has made millions of electronic devices fail due to failing capacitors, even lately, when a manufacturer tries to go cheap and use Chinese capacitors. They leak and short out, even though many manufacturers have quit using anything but Japanese capacitors, which have distinctive markings. Look up images of Japanese capacitors on Google.