I used my Foxwell scan tool in Live mode on the way to work this morning to monitor the Voltage from the O2 sensors. I don't know a ton about this but here's what I was seeing.

O2B1 S1 was bouncing all over the place. If I let off the throttle it would go to 0volts. Otherwise it appeared pretty erratic.

O2B1 S2 seemed more stable but did drop below 0.1Volts sometimes.

I had been getting the P0420 error for a while after adding a Milltek header and tune. Installed a defouler and now I don't seem to be getting that error anymore, but I am getting P0136. I started to think maybe I have a broken O2 sensor, but assumed it would be the post cat sensor, but I'm confused about these voltage readings.

 

Closed Loop o2 sensor control is used to control the amount of fuel to inject into the engine. So it is constantly adjusting for the driving conditions. It goes to 0 volts because you are off the gas, no air is going into the engine, so it needs no fuel. And it is just pumping air so it reads lean. Take a look at the picture below.

 

What real time voltages would I see if I do have a faulty O2 sensor that isn't completely dead?

I was seeing some readings less than 0.1v on secondary sensor which by the graph looks like a lean mixture. I thought it would have been rich based on the tune and Milltek cat?

I also saw some higher readings at least over 0.8v on the first sensor too.