I was getting little to no heat in my 2001 Catera at all so I chased all the relevant threads and formulated a plan of attack.
After much flailing about and wailing and gnashing of teeth it turns out the biggest issue was a bad HCV valve and a clogged heater core.
Given all the issues many have posted with the HCV valves I ended up swapping out the HCV for a 5/8 brass T for now. Will consider either replacing it with a new HCV or a valved T or Y connector so I can manually turn on the flow to the core for the Winter months and off for Summer. I began to get a small amount of heat shortly after doing the swap, but not enough. I flushed out the old Dexcool until I had pure water and added a bottle of Blue Devil radiator flush. I ran it for about an hour and a half including a 25 mile round trip to town. While driving the heat continued to get better until it was quite hot. Radiator temp was just shy of 50% on the gauge.
Drained and flushed again...and drained the radiator and reservoir as far as they would drain. I added 6 quarts of Prestone Platinum to the water that remained in the heater core and the engine and that topped the system up completely. After running another short drive to force the air out of the heater core I now have seriously "like new" heat on the driver's side, but sadly not the passenger side. I will check the activator rods under the passenger side sometime this week and see if I can find the correct operating rod to zip tie into the correct position to open up the mix valve flap for the winter. Heat will then be controlled by fan speed. Even without that the car gets plenty warm...
Gonna have to leave it like that for the winter.