Some guys have mentioned that it seems to take a day or so for the headers to make a difference. I find this doubtful too but could it be from the O2 sensor being temporarily dirtied or confused by the coatings from production on the insides of the pipes? Also, thanks for your work on the cooling system. Let us know how the new thermostat works.

 

If the O2 sensor were to get dirty and not fail there would be a delayed response and would probably set a DTC. The switching O2 sensors are expected to have a certain frequency, significant variation from this will cause a code to be set. But I guess it is possible...

However, the heated O2 sensors used do burn off and contaminants very quickly so it is unlikely that any residue would cause a problem.

On my car I had the laptop hooked and the fuel trim values adjusted a few points rather quickly indicating that the O2 sensors were reading a change.

 

OK Thanks...