Steve, I have tried several known good ECUs. I have the luxury of four 700 series Volvo's at my disposal as parts sources. Are there some tests I can run at the ECU that point to the knock sensor requirement? I do not know of any.

Chrism was correct. I have to re-set (clear the code) each time. I have done it by the book and I have done it by disconnect of the battery. Results are always the same - it will repeat. I'm almost sure it is not the sensor, but elsewhere - but where is that? As to unbolting the sensor I have done that too. It was cushioned and secure. But if I understand the sensor it must be in the circuit so the correct signal is available as rpm approaches 3000 (it is an auto test of circuitry at that point - I'm told). As to a static charge, I have done that too - I drive on Michlens exclusively and switched the tires from car to car to check. Maybe I nedd a "slap" bar before I enter (just kidding). I still think I have a wiring problem - but where?

jmars suggests I may have an internal engine problem. I do not want to think that - Yet! The engine is sooo smooth and level. Not a sound of valves or slaps or wrists etc. I'm baffled! If it is the ECU then the three I have used are all faulty and should manifest the symptom in the other cars - but not so.

I appreciate all the comments. Brainstorming may be the only way at this point. Thanks for the help - to all.