P0420 with dropping exhaust temperatures and lean fuel trim

I hate to be that guy who asks about cat codes for the millionth time, but because so many smooth brains like me ask dumb questions I haven't been able to find a thread with similar problems as mine.

I have read shifty's links on P0420. Here's some background info:
- Upstream o2 on both banks appear to have the normal fluctuations between lower and higher voltage
- Downstream o2's mimic the upstream. Which they are not supposed to. They are supposed to read pretty level/ flat. This usually points to either bad sensor, wiring, or cat.
- Exhaust pipe temperature before the cats on both banks is slightly above 200F, and after the cat it is around 180F. Also not supposed to be this way, usually means the cat is bad.
- Long term fuel trim is 14-16% on both banks. (running lean?)
- Short term fuel trim is right around 0% on both banks.
- Knocking on the cats with a rubber mallet doesn't lead to any rattling.
- In the colder months most startups would have quite a bit of smoke, which I have heard can contribute to cats going bad.
- System is operating in closed loop at operating temperature.

O2 sensors will ordered, but I'm thinking I've probably got more going on as well. Thoughts? Seems like there may be an exhaust manifold leak, but I am here to ask the experts.
Will update.

 

420/430 codes are notorious for being miss diagnosed.

Those 2 together begs the question what physical condition does your driver side oxy sensor look like. We've seen some rusty as hell looking sensors.
If you have exhaust leaks after it exits the engine or after the O2 sensor, you have unmetered oxygen which can cause the rear O2 sensor to follow the A/F sensor.

That being said, is 420 the only code or have you seen 430 also ?
Is your reader showing the 420 as pending or stored code ?
Hard to agree if you're saying you are in closed loop and normal operating temp.

Is that right after driving for a bit and the engine is running in idle ?
Also, are those indeed F readings and if so, what is your coolant temp. Makes me wonder if those are C readings given 200C = ~400 F. It is somewhat true that the rear should be hotter than the front but not totally true. Are these original Cats ?

What color and for how long. How is your oil and coolant levels....having to add to either ?

More questions than answers but not convinced Cat or Cats are bad.

 

Sensors all looked pretty fine, I didn't pull them out to check the connections though. The truck is rust-free. There was some weird zip-tie wiring going on at the passenger side O2 though which makes me think I've no longer got the stock sensors, and they probably weren't replaced with denso.

When the 420 first showed up it was a stored code, with 430 pending, but then the next week when I had time to really look at it again it was only a stored 420, and the 430 was gone.

Yes those are Celsius, my bad.
Yes it was at operating temperature, at idle. Hadn't been driven for 30 minutes or so though, just idled it back up to temp.
I have been assuming that they are, but I do not know if they are original or oem. Is there a quick visual that can confirm?

Usually white smoke for maybe a minute or two. I have seen blue, but only once or twice. Oil, coolant, and power steering all basically stay the same, don't have to add.

Thanks!

 

I'm not suggesting to run out and spend $60 ea for sensors but if it was my Tundra, I would be changing them out as soon as I could. Same for any other maintenance you might not have done so you can create a new baseline of whats been done. Buy from dealer or at least from reputable source. Surely you've seen comments concerning Amazon and Ebay knockoffs.

Denso sensors 2000 AC SR5 4x4.... Search Results | DENSO Auto Parts....

With your 2000, the harnesses for the rear O2's should be same link. For kicks and grins, swap left side to right side and vice versa. If left is indeed bad or squirrely, code then should follow and be 430 code. At end of day, you need to consider changing all four for reasons already mentioned.

As far as possible exhaust leaks, I have used a shop vac with outlet side to your tail pipe with cold engine and off....leak will show bubbles.

 

Sounds good. Managed to get upstream denso sensors at rockauto on sale for $26 each. Downstream mention coming with a gasket but the upstream don't, do the upstream need a gasket as well?

 

The upstream simply screw in and should come with a dab of anti seize applied.