here is how i fixed my problem. from searching i got the feeling others have had similar trouble. with some references from this site i was able to figure out the rest of a simple repair. i am posting to save others some time and confusion.
recently i have had frequent SUDS errors even after reducing amount of soap, and some residual soap in dispenser. i watched the wash and noticed no soap in water at start of wash. opened dispenser drawer and saw water passing through rinse and bleach side of drawer on right. during rinse cycle, water passed thru soap side of drawer.
turns out that over the years some soap crud built up under a actuator arm that is spring reset and the spring could no longer overcome the increased resistance. boiling hot water and a toothbrush cleaned it up, reduced the friction, and restored normal function.
UNPLUG machine from electric power.
remove dispenser drawer.
remove top of machine. Remove the top by removing three screws at the rear of the lid and sliding the top toward the rear of the washer about 1/4 of an inch before lifting.
looking down from in front, at the front left (above the dispenser drawer), there is a lever arm that can point at one of four positions labelled roman numeral I - IV. a spring pulls it to the left. to the right of the lever, there is a cam which pulls the lever to the right to place in the correct position for cycle. aroung the cam there is a tiny indicator arrow and there are tiny roman numerals I - IV to correspond with the numerals for the lever arm.
disconnect the spring from both ends and set it aside.
the lever arm should swing freely. manipulate the lever and look in the empty drawer slot and see that under side of the lever arm directs water from left to right to correspond with prewash, wash, bleach, rinse (I, II, III, IV). in my case the arm moved easily but not easily enough for the spring to pull it left due to soap crud. i set small towels to prevent water from dripping down off the top of the dispenser, dipped a toothbrush in boiling water, and scrubbed away the crud. much of the water passes thru to the drawer so there is little for the towels to catch. very important to turn brush sideways so bristles go under the lever axle and work it all around. now the lever should turn MUCH more freely and the spring should easily reset it to the left as far as the cam will allow.
reattach the spring. note where the cam is pointing (small roman numeral) and set the lever to same position (large roman numeral).
replace the top.
replace the drawer.
replace power cord.
Source: whirlpool duet washer GHW9200LQ dispenser