Oxygen Sensor or other problem

2003 Astro
RWD
158 000 kms

Van ran perfectly till Monday night. Wouldn't start. Tried till it flooded.

Next day same thing so I changed cap and rotor. Started on first try. Ran REALLY bad and RICH. Checked plugs on drivers side bank ALL black. Plugs on passenger side bank were in really good shape.

Changed out all 6 plugs for new along with new wires.

Now van has new plugs, wires, cap and rotor. Starts up and runs nice but still has slight shake periodically at idle and solid engine light.

Called dealer. They say the reason plugs are all black only on drivers side bank is because the upstream Oxygen Sensor is faulty only on the drivers side. They said they could do a diagnosis for 100 dollars but I would be better served just changing the Oxygen Sensor on the drivers side manifold as they are 99% sure that's what it is.

Any other possibilities anyone?

Sounds more like a problem with excessive fuel pressure leakdown. Possibly a leaking fuel line or fuel pressure regulator, or even a fuel injector. All are under the upper intake, in the plenum. If you have an Autozone, or Advance Auto Parts in your area, stop by and have them retrieve the DTC(s), (Diagnostic Trouble Code). They will check it at no charge. Write down the DTC's in the order they appear and post them here. The DTC's will start with the letter "P" and have 4 digits following the "P". Don't buy any parts yet. We can help you diagnose the problem so you don't buy unnecessary parts. BTW, lose the Haynes manual, some of the information in it is not reliable.

We don't have either of those stores here. I'll look into a place that does free tests tomorow I haven't heard of one in Canada.

If I can't find one then I'll change the Oxygen sensor anyway cause its the same price as a test and it's part of maintenance at 100 000 miles anyway. if the code and rough idle is still there ill do a test and see whats left to throw money at.

Do these van EVER get more than 15MPG HWY?

My wife's 2000 AWD will do over 19 highway at about 70-75 MPH on a 360 miles cruise to our cabin. It has 3.73 gears, too, so one with a 3.42 might do even better, and the AWD has to cost some mileage.