I have a 95 civic which I bought with a dead hole. I pulled the head, cleaned it up and replaced #4 exaust valve. Tried to smog (ca.) but was too rich. Told I may need a cat or valve job but I didn't buy it. I ran injector clean through two tanks of gas, cleaned throttle, ran full bottle of seafoam through intake using a special metering device, adjusted valves and replaced plugs. This time the HC's were worse ( only slightly over the first time, don't have #'s in front of me but everything else passed). Bought a cat and replaced it, then thought I had better look at o2 readings. ( I know, bassackward ) I found that the o2 likes to level out at idle right around 640 mv and raises with rpm. O2 sensor will respond rapidly to input, ( goes lean with vacuum open and rich with carb cleaner or vacuum off fuel regulator). But will not switch between 100 - 900 mv. ( This is all after warm up ). I checked the coolant temp sensor and it shows about 2700 ohms cool and 247 ohms hot, which is good. I also checked for intake and exaust leaks and found none. I have not as yet checked the map sensor, frankly because I haven't found it yet.
SO... My question is this: Why is my o2 sensor not showing any swing. As this is a function of the ECM, is there a test that will identify an ECM issue?
Thanks in advance

The O2 sensors only report the conditions that they "see" in the exhaust stream. The ECM makes that data available on your scan tool.
Excessive HC is simply unburned fuel. If the fuel isn't being consumed inside the engine, the converter is designed to clean it up before it can go out the tail pipe.
A tired engine along with sensors that may have seen better days can cause excessive HC.
Thorough analysis of your engine and it's operating parameters is the best bet here.