Hi ssam1

I'm Harvey the Master Plumber.

You want to know if you can fix a sudden low volume /pressure delivery problem on an American Standard model AS4205100 H7, one hole / single lever with pullout spray And is it fixable by owner?

Yes and maybe!

All plumbing systems are plauged with debrie buildup. Every time you turn on a faucet the water starts moving and will bring bits of debrie up from the water main or well. Most are small and will pass through to be caught by the screen in the aireator. Sometimes a relatively large piece comes along and dosen't make it through to the screen. It is stuck in the body of the valve.
Fixing takes a little mechanical ability.
First, turn off the water to the faucet in question. Then carefully disasemble paying close attention to the order of disasembly so you can reverse the order when putting back together. As each part is removed inspect it for that troublesome bit. I find that a little bit of wire or a paper clip is helpfull. Carefully reasemble. If the volume is still low then you missed finding that bit. Try again or call in a local pro.

Feel free contact me again!
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Thank you,
Harvey your Master Plumber