Oxygen Sensor

On my 3.0 DOHC, I'm receiving a check engine code P01555 stating my upstream oxygen sensor has a fault. Watching the signal from it, the voltage signal rises very slowly when the engine is started, then the signal swings back and forth correctly for a few minutes before settles out around 7 volts with very little swinging. To make sure the heating coil was working properly, I wired it from a seperate source to make sure it is heating but still get P01555 fault. I tested the heater off of the engine and it does heat up. The other upstream sensor is operating properly. Any idea why it is acting this way? Problem doesn't seem to occur during warmer weather. Just around and below freezing.

Sounds like you got a lazy 02 sensor. I'd replace it and see what happens. Also, are you getting your measurements from a scan tool or from a voltmeter? A voltmeter commonly gives you an average on quickly changing or pulse-width readings.

Thanks for responding. Using scantool software on my laptop PC. Had already bought and installed a new oxy sensor with similar sluggish results. Maybe that one is bad, too. Checked out mass flow sensor in the meantime with a voltmeter and it seems to be working properly.