[kressler]

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.