yup, at idle only two cores run, during a stress test they all ramp too 100% my aio has been used for 2 years i got it off a family member
Just 2 years? Seems a bit short, but AIO/CLC lifespans are all over the place. Less than a year to as long as... I think the longest I've heard is 9 years? Though some of that is influenced by how much time the unit is run daily/weekly/etc.
There's not a convenient means to fixing them if they fail, so prepare for possibly replacing this... what's left is checkups to see if it's really failing.
For one, have you removed the cpu block and examined how the paste spread over the cpu?
When you repaste the cpu, manually spread the new paste over the entire IHS. You can use an old credit/debit card, one of those disposable clear gloves - even wrapping a small section of plastic bag around your finger works.
When securing those 2 screws, use half-turns, and go back and forth between them until they stop.
Second, air sometimes fails to get pumped away from the cpu block - you can give it a hand.
Once you've completed cooler installation, tilt the case backwards while the PC is running for about 20 seconds. This helps guide any air pockets in the cpu block to the radiator.
Third, load up Cinebench R23's 10 min thermal test. While it's running, feel both of the tubes.
If fluid flow is good, it should be hard to tell the 2 apart. If flow is poor, it'll be clear as day and night.
If it doesn't check out after all that, then RIP, I'd say...