Hi Alan, You're probably connecting the scope(or meter) ground to the battery -ve. Try it on the O2 sensor itself. If you then get switching, as previously said, it'll be an earthing issue! I once looked at a Volvo with a new sensor stuck @ 0.5V. The O2 'boss' had been knocked off during a gearbox change and welded back on. I instructed the garage to check that the sensor tip was well into the exhaust stream, and that the welding was air-tight. They did this and the fault was cured. I believe this type of O2 sensor is a voltage generator, so should switch even if not connected to ECU!! Hope there'e something here to help, Regards, Nick Bell