2004 Accord 2.4l with about 250k on the odometer. In the past, I was getting one of those O2 sensor codes(can't remember which), but the code would come and go. I was able to pass the CA smog test, but figured I'd replace the upstream O2 sensor because of the mileage and because of the fact that when I'd look at the live data for O2 sensors, I'd get zero volts on the upstream, but about .6 volts on the downstream. Figured it couldn't hurt to replace it with a Denso unit. Well, I'm still getting zero volts on the upstream O2 sensor and my question is this: Is this normal? I thought they both had to show voltage especially when reaching normal operating temperature? If so, then I might start looking at the wiring harness for the sensor for "opens". Also, it has the original Catyltic converter. Thanks for any help.