First, I would quite dumping in chemical cleaners. They can sometimes make things worse. It's like using to much nasal spray for allergy suffers.So I’d been chasing a misfire issue for a while now and after running a couple of cans of SeaFoam Top End Cleaner through my intake manifold the misfire issue has disappeared. However, not long after running those 2 cans through, I’ve been popping code P0430, which is the post cat O2 sensor reading “Catalyst System Efficiency Below Threshold (Bank 2)”. I did some online searches and looks like if the exhaust is hotter after the cat, that the cat may be fine. Pre cat, the exhaust is around 230-240 while idling, and post cat, it’s right around 400. That seems to indicate the cat is OK, correct? The car seems to drive fine, but while idling a do hear some stuttering out of the exhaust. Could this be a rear O2 sensor? I thought the only thing they did was monitor emissions. The front 02 is what tells the engine how to run correctly, but I’m not getting any pre-cat codes. Should there be more than a 160 degree or so difference in the exhaust temp pre and post cat indicating I may still have a cat problem?
Thoughts and/or suggestions?
With the age and miles, O2 sensors do degrade. They can become erratic or slow to respond like they should and this can cause errors.
It is not a bad idea to replace all the O2 sensors after an engine reaches higher mileage.
If you can get or have a good OBD II scanner that can read live data, it may help to see what the O2 sensors are reading.
Bu I would just replace that after cat O2 sensor for a start.
Some replace all of them after 100K miles or so just as general maintenance. It can also return better MPG if they have gotten "lazy" and slow to respond to exhaust stream changes.
I had one slow O2 sensor on my 2015 3.6L V6. MPG improved +2 MPG after replacing it.
A slow responding O2 sensor may not post any errors until it gets quite out of spec.