Beautiful car. If you haven't replaced all three sensors, I'd do them all at once and clear your codes. If you have a legal cat in your midpipe I'd be surprised if that code came back.

 

I know that the Manzo cat back exhaust isn't all that loud with pre-cats removed from the stock header. The sound is slightly louder than stock and sounds more like what the exhaust tone should have been from the factory. I believe there are Spyder videos on YouTube with that exhaust if you want to compare sound.

You may have an exhaust leak causing the louder sound than typical for the setup you have or a loose O2 sensor/wd40 damage. I had to replace both upstream O2 sensors when one went out after changing out my exhaust. It's been fine for 10k miles so far since the change-out.
Another thing to try would be disconnecting the battery for a bit so the computer resets for the new setup and sensors. You've probably already done that but just throwing that out there.

 

If I had to guess I would say prolonged oil burning killed your main cat and it was simply the Precat material keeping your P0420 at bay. Now that those precats are gone, your cat is failing. My main cat didn’t make it to 200k and even on a completely hot engine Precat made the difference between smog pass/fail on the sniffer test so they do have a marginal effect on exhaust gases.

Anyway, does your car burn oil? I’m only assuming it does because it’s a pre-facelift.

 

If I had to guess I would say prolonged oil burning killed your main cat and it was simply the Precat material keeping your P0420 at bay. Now that those precats are gone, your cat is failing. My main cat didn’t make it to 200k and even on a completely hot engine Precat made the difference between smog pass/fail on the sniffer test so they do have a marginal effect on exhaust gases.

Anyway, does your car burn oil? I’m only assuming it does because it’s a pre-facelift.

I keep meaning to check the oil 😅. In regards to the 1st half, My exhaust (stock) had no issues. I only replaced it because of my concern of failure (of the pre-cats). My main cat (now) is brand new (new downpipe). Ideally, if it works as it (the new cat that is) should, I should have low enough emissions to pass. What I'm thinking is my father in law doused the O2 sensor in WD-40, and now it's failing. I don't know / remember, which one he sprayed, he may have sprayed them all for all I know. What I'm particularly interested in, and I will do some digging this afternoon/evening, is how to find out which sensor is bad, or if there is even a way to tell. (thinking checking with multimeter through wiring diagram and electrical theory, etc.) I assume all o2 sensors will need replacing... 😔

 

Technically I don't think most high flow cats are road legal (not that they won't pass emissions testing but just by the letter of the law). Either way, spraying down the sensor with anything short of MAF/electrical contact cleaner is a definite no no...hopefully that is the extent of your issues.

FWIW the stock setup is somewhat restrictive but the huge upside is how much that lends to volume control. Best case scenario is you may just need to replace the contaminated O2 sensor.

 

Exhaust leak.

 

I will try again after work, but I cannot feel any air escaping from any joint in the exhaust. I checked when I got to work. I remember the first time I started it up after I installed the new exhaust and I had an exhaust leak at the downpipe to cat-back, I adjusted the joint, re-tightened, and the previous air flow that existed before, no longer exists now. I've done a 360 degree check of all joints. Block to header, header to down pipe and down pipe to cat back. 100% no exhaust leak. I can check if there's a leak when I start it up cold. I'll check all 3 joints again. Not sure how it would be the same [or nearly the same] volume as a stock exhaust. It was previously 1.75" and is now 2.25" plus 1/2 the [high-flow] cat [1/3 technically, but each cylinder's exhaust only goes through 2 when stock, now goes through just 1], and the muffler is less than half the size of the stock muffler, again with much larger piping [inlet/outlet] with a 3" tip for the last 6 or so inches.

 

Found this wiring diagram, hoping it will provide useful info.

 

1. To test the heater, disconnect the sensor connector.
2. Using an ohmmeter, measure the resistance between terminals +B and HT:
   • At 68°F (20°C), resistance should be 11–16 ohms.
   • At 1,472°F (800°C), resistance should be 23–32 ohms.
     

     ​

     Heated oxygen sensor terminals

3. To test the sensor, connect the OBD II scan tool or a Toyota hand-held tester to the data link connector (DLC).
4. Warm up the oxygen sensor with the engine speed at 2,500 rpm for approx. 90 sec.
5. Read the sensor voltage on the scan tool screen as you perform the following tests:
   • At idle, the A/F sensor output voltage should fluctuate 6 or more times in 5 seconds.
   • During fuel enrichment (acceleration during a snap-throttle test), the output voltage of the A/F sensor should be below 0.56 V on a generic scan tool (2.8 V on the Toyota tester).
   • During fuel cut (deceleration during a snap-throttle test), the output voltage of the A/F sensor should be above 0.76 V on a generic scan tool (3.8 V on the Toyota tester).
   • If the output voltage of the A/F sensor remains at 0.660 V on a generic scan tool (3.3 V on the Toyota tester) even after performing all the above conditions, the A/F sensor circuit may be open.
   • If the output voltage of the A/F sensor remains at or above 0.76V on a generic scan tool (3.8 on the Toyota tester), or at or below 0.56 V on a generic scan tool (2.8V on the Toyota tester) even after performing all the above conditions, the A/F sensor circuit may be shorted