From o2 sensor to engine replacement in 10 days?

You had dealer install a cat for $2000+ and it's still loud? Something doesn't seem right, something is leaking from the manifold to exhaust

 

I understand the car has logged 170K miles.

Are any warning lights on now?
When was the last time the iridium spark plugs were replaced?

A good idea to clean the mass air flow sensor (using MAF sensor cleaner spray) and the throttle body interior (using a clean cloth sprayed with throttle body cleaner, do not drown the throttle body in cleaner fluid.) Remove the air cleaner housing for access to the throttle body interior.

 

When you take the power away from the computers, they have to “relearn” how to behave or act.

Some will do this quickly or with a short number of cycles, others take time.

 

In case you are not aware of the MAF sensor location, it is secured with two little Phillips screws to the air cleaner housing. You need to remove the sensor for cleaning.

After it is removed, you will see an amber bulb (which may be grey if covered with dirt) and two very fine wires. Do not touch the wires with anything other than the cleaner spray. Those wires provide the sensing capability of the air flow.

You will notice a little rubber O-ring which seals the sensor to the air cleaner housing. Make sure that is in good condition before you reinstall the sensor.

After you clean the MAF sensor and the throttle body interior, make sure that the ducting which connects the air cleaner housing to the throttle body is tight and secure. You do not want any air leaking into the system downstream from the MAF sensor.

If the air cleaner filter is dirty (look at the bottom of the filter to see the dirt) then replace it with a new filter. If the new filter is not Toyota-branded, make sure that it fits tightly so that no dirt can bypass the filter. Make sure that the positive crankcase ventilation (PCV) hoses are tight and there are no leaks in the hoses.

You asked about the engine ECU having to learn. The reason that the ECU needs to learn is because there are no adjustments with respect to engine idle speed, engine ignition timing, fuel/air mixture, etc. Each engine is slightly different with respect to throttle plate resting position, the amount of internal wear, etc. Hence the engine ECU will make these adjustments based upon its various sensors. This needs some time to occur after 12V power is restored.

 

So, after you've done the cleaning, put the engine intake back together and take a relatively long drive on the freeway, 20 miles or more, assuming no warning lights appear. Then see how the engine behaves driving around town.

 

Well, see how the car performs as you drive around your neighborhood. I don't want you to get into trouble, but you need to give the car a nice run to assess its performance after you've done all this good work.

 

Just take it for a spin down the Dallas Memorial Highway and head toward Wilkes Barre. Let your significant other know your path and have your cell phone in case you run into issues.

While there is some gradient, getting a good run to assess the situation and allow the computers to relearn their settings is important.

But there are many ways to get to the city from Dallas but the cross valley expressway will be a good run.

 

The learning part is no big deal, all you have to do is drive the car at various speeds and it does it on its own..
Once the 12 V battery is disconnect you are doing a hard reset of the cars computer. This clears all the codes in the cars computer but also all the things that it has learned. Once you start driving it again and complete what is refered to as a Drive Cycle the learning process is complete. The computer in the car has sensors like your O2 sensor, MAF and a bunch more that are all inputs to the computer, If one of them gives bad info sometimes the computer just does what it's told and the car runs bad.

 

Before @theelectricme comes on here, we are almost at 200 posts

Getting back to the subject, after you clean the maf sensor and throttle body, go for a good 20 minute drive to warm up the car. See how it performs.

You might have to disconnect the negative battery terminal for a minute after cleaning the MAF sensor to reset the car. If you don't, you might get the sensation that your engine will not idle properly and want to die

 

Let's put this on the back burner until after your cleaning tomorrow.

Do you have the paperwork with the parts replaced? You can ask for the dealership service manager to address the issue if it persists, they should have a shop foreman check the installation. It should not be loud, if it is, there's a leak.