I finally got a chance to work on my car last weekend. I dropped the exhaust and inspected it for any holes and found nothing wrong.

I noticed that the gaskets on both sides of the intermediate pipe were worn down significantly. There was enough soot on the outside edge that exhaust gas to indicate some exhaust was escaping from there. That also means that outside air could seep in and cause the O2 sensor to misread the oxygen levels. I replaced the two gaskets on the intermediate pipe, and the two gakets where the down pipe meets the exhaust manifold.

The bolts with springs were not rusted on the intermediate pipe, but I had bought new ones any way. If someone else tries to do this, just buy the gaskets and the springs. The mesh on the catalytic conveter looked fine, so I guess the catalytic converter is ok. The bolts on the gasket past the cat were rusted together, so I decided to leave that one alone. I installed the new O2 sensor (eventhough the other one was ~3 months old).

I am at a half a tank after driving 160 miles, so I will assume that the problem is fixed.

It makes some sense why the code 43 popped up when the temperature was cold. The exhaust seals weren't warmed enough to make a good seal allowing outside air to enter.

I will post an update my gas mileage after a few more tanks to verify that I solved the problem.

Thanks for all of the suggestions.

 

Here's some geeky explanations for part of it...

It's not real plausible for air to leak IN while exhaust is leaking out. But the O2 sensor measures O2 DIFFERENCE from inside to outside. So exhaust blowing around the OUTSIDE of the sensor will cause it to read the same as excess O2 inside. It'll think it's lean, so the ECU adds more gas.

You can also screw up the O2 reading by plugging the vent holes in the sensor exterior with oil, crud, sludge, etc.

 

The explanation I have seen is that extra fresh air is pulled into exhaust system during lower pressure (subambient) phase of exhaust cycles between cylinder exhaust pulses.

Your explanation has some validity also, but there is a lot of air for escaping exhaust to mix with.

regards

 

That's very nice. Thanks for your help.