I don't think it's the Techron cause I have used it in a couple of my 240's with no problem. You could have a bad 02 sensor or you might have a plugged up Cat. The other reason could have gotten killed would be if you had a blown head gasket and the coolant got into the exhaust and will kill any 02 sensor new or otherwise. To clean up the injectors in the future one of the best products on the market besides Techron that I've found is "Redline" injector and system cleaner. It is good for 100 gallons or use the whole thing for a quick cleanup of the whole system. One whole container in the tank. I had an 86' 240 that was really runnning rough. And after the "Redline" treatment it was smooth as silk. Hope it works.

Bill