... only the ones that use coolant fluid. Most of them don't. My G5 has almost 5 years strong and the only failure so far was a weird brown pillow shaped capacitor in the power supply.
My mom has a used iMac G5 that runs fine. I think for any reasonably well used iMac G5s that capacitor plague would have been exposed by this point in time. Not all of them had the bad capacitors and sometimes Apple will still fix the problem for free if you bother them enough.