Your above my pay grade but I found this video to be helpful.
With that many miles it could be the cat/exhaust system in general.

 

Wow...I actually learned something today! Sincere thanks for the link...I've watched that guy's videos before, I like the way he spells things out in a way someone not-so-smart (like me) can understand. ;-)

So my short term fuel trim looks good...I think I'll just keep an eye on it and see if the long term fuel trim begins to come down. Wondering if the extended driving on a bad (or questionable) O2 sensor may have caused the long term trims to be that high...and if so, would driving on the new Denso sensor cause the short terms to become okay and the long terms to eventually come back down to normal. At least...that's what I think I've gathered from the video. And if that doesn't work, I'll go back to square one and rewatch the video for another attempt at knowledge...heh...

Thanks again sdspeed!

 

Which O2 sensor did you replace? There are two - one before the catalytic converter (primary) and one after (secondary).

The primary O2 sensor is used by the ECU to monitor exhaust gases and adjust fuel trims to ensure a proper fuel/air mixture.

The only purpose of the secondary O2 sensor is to monitor the condition of the catalytic converter, The ECU compares the signal from the primary O2 sensor to the signal from the secondary O2 sensor and throws up a P0420 code if it determines that the catalytic converter isn't working properly.

P0420 typically means one of the following:

• primary O2 sensor has a problem, though other performance issues will normally occur if that's the case, or
• secondary O2 sensor has a problem, or
• catalytic converter isn't working as it should.

At your mileage I would suspect a failed catalytic converter. The chemical reaction in the converter is an exothermic reaction, to the gases leaving the converter should be hotter than the gases entering the converter. If you have an infrared thermometer, one easy check is to measure the temperature of the exhaust pipe a couple of inches before the catalytic converter and a couple of inches after. If the temperatures are roughly the same, the catalytic converter isn't working. You want to do this when the engine is at proper operating temperature.

It seems to me that your long term fuel trim is a bit high. That could be a symptom of a bad primary O2 sensor. It could also be a bad MAF sensor or a leak after the MAF sensor in the intake.

 

Thanks for the info kenny-bob.

Sorry I didn't specify... the front/primary O2 sensor is the one I've been replacing. First with an off-brand, then with a Denso.

Before diagnosing a miss as the fuel pump problem, we did replace the MAF with a Denso unit... so at least we can rule that out.

I do have a thermometer I can use to measure exhaust gasses temps... I'll do that and report back.

 

Take car to muffler/cat shop. Many will (should) diag your cat for FREE. They will drill a couple holes in your cat--pre-and post--and measure the pressure. They should be the same and w/i limits (2lbs? if memory serves). They will plug up the holes they made and/or sell you a new one if needed. That check should be free, so no $$ out of your pocket. If they warn they charge when you call shops up, call the next one. You should only have to pay for the gas to get to the shop to have the cat tested.

 

Sounds like a plan... thanks much!