Codes PO420 and PO171. Where to start?

2000 Tacoma. 3.4l v6, 5 speed, 4x4, 128k miles. 50 States emissions (yippee...), but I live in MD, so it's not necessary.

When I bought it, it had the code for running too rich. It was awful. It got about 9mpg and reeked of gas. I replaced the front O2 sensor (with a the PROPER denso sensor)

Replaced the front O2 sensor and it immediately ran a LOT better. Instantly got 16 mpg (which I think is still too low) and didn't reek of gasoline all the time.

200 miles later, the CEL popped up again. It was for PO171 I believe. So I cleaned the MAF sensor and reset the codes.
500 miles later..it came back.
From this point on, I would reset the code, and it would come back more and more quickly.
It is now to the point that the PO171 code, and now PO420, will come back on within about 15 miles of being reset.


Known issues with the truck:
Gets 15-16mpg.
Lacks power overall. Certainly does not feel like 190hp (my 1994 Nissan hardbody with 153hp had much more "umph" with similar gearing). and at 90-100% throttle, it bogs.
Codes PO171 and PO420
Sometimes I hear rattling in the exhaust when the truck is cold. So I attributed this to the cats falling apart.
truck is kinda noisy. Has a high level of noise from the engine bay (and overall) for a 2000 vehicle, especially a Toyota. idk if this signifies air leaks, or valve issues or what.

What I have done:
Tested voltage for rear O2 sensor and it was in the acceptable range.
Observed temp of both cats. They were both far below acceptable range of operating temperature.
Checked for air/vacuum leaks to the best of my abilities. I originally sprayed a bunch of starter fluid around certain areas of the engine with no results. Then I discovered a method using a propane and an extension hose. Also no results or changes in engine speed.
I also took off the entire air intake assembly and used water to check for leaks. It did not leak a single air bubble.

My issue is that I do not want to spend a bunch of money on both cats if they won't fix the issue. When researching PO171 and PO420 codes, it comes up with SEVERAL different causes. Fuel injectors, fuel filter, EGR system, cracked vacuum hoses, exhaust leaks, etc etc.

I do not know where to start especially since a lot of the diagnoses can required involved work (getting to the injectors for example)

Any help would be appreciated. Thanks

 

when you measured the temp of the cats where did you measure(before cat, center of cat, or after cat)? and was it at operating temp?

 

I think I measured all 3.

It was a while ago. I watched a video on how to do it and the idea was that the exhaust AFTER the cat should be substantially hotter (200-300+ degrees F higher) than the exhaust pre-cat.

I believe my exhaust was barely 50 degrees hotter, if at all. I should re-do the test.

 

when 171 popped on me it turned out to be a loose hose on the pcv line near the back valve i had running to my oil catch can. It lost its vacuum. Might want to check that line out.