Issues with Magnaflow Catalytic Converter

I recently replaced my cats after seeing the dreaded P0420 code. I'm in an area where pollution is an issue so I really needed to replace them. Went with the Magnaflow cats for a 2001 Tacoma 5VZ xtracab. The upper cat is HM grade and the lower cat is OEM grade. I had a shop install them and set them up. Recently the check engine light came back on reading the same code. They were able to clear the code but said there is a chance that the light would come back (which it did) because the cats were incompatible. They also mentioned that they would again be replaced with cats directly from Tacoma. Now I already forked out ~1500$ for the first set of cats and I really don't want to replace them again. Some other issues I've been having that may or may not be related is my gas mileage has gone down the tube. Now I know it's a V6 but I use to be able to get 18-20 mpg but now I only get about 13-14 mpg. I can also tell the truck doesn't appreciate being on the highway for more than 10 minutes (smells warm if that makes sense). Also my MAF sensor may need to be cleaned as well. With those issues I'm just curious if anyone has an suggestions on what I should do and if any of these issues could be related. I want this truck to last a while so I'm trying to do the most preventative maintenance. I've had the truck for about 8 months if that helps. When initially purchased there were no problems.

 

Before replacing the cats, was the downstream o2 sensor replaced?

 

No it was not.

 

Might want to start there. Use Denso.

A bad downstream (after the cats) o2 sensor is typically the cause of a P0420 code.

 

Another common cause of P0420 is exhaust leaks. There are a few ways to check. My preferred way is to set a shop vac to blow (usually by swapping the hose to its exhaust) and affixing it to thee tail pipe (use tape for a good seal). Turn the blower on and spray soapy water on every seam in the exhaust. Look for bubbles to indicate a leak.

Before doing anything else, though, clean your MAF. It's easy and cheap to do yourself. Plenty of how-tos already on the internet if you search.

 

I have been running Walker Exhaust Cats in all my Toyots's for 20 years with no problems.
Exhaust leaks and bad rear sensor the only real problems.

Something doesn't sound correct here.
Good luck

 

I'm currently chasing the same code so I have read a lot about P0420.

This code is only thrown if your waveform of the rear oxygen sensor mostly matches the front oxygen sensor. This then tells the engine the catalytic converter is not doing it's job. A common cause for this to happen is a leak between the front and rear oxygen sensor. The rear oxygen sensor cannot account for this and thinks the system is lean, which will then dump more fuel in.

Check for leaks between the sensors first. If you already replaced both cats, I would think the leaks would be solved, but who knows.

 

Take a moment and watch this. He talks about why the lean can happen. That shop cleaning the throttle body and stating it was done to correct the lean condition, yeah to me that is questionable. Exhaust leak is a definite possibility. Before replacing the sensors, check their gasket (lean condition).

https://www.youtube.com/watch?v=ww0ottmbzes

 

California may be more expensive, but that doesn't mean you have to go OEM on the cats. It's either your cat, or an O2 sensor (or other air/fuel related things) . $1500 is way beyond what this should have cost so far

 

Oh yeah I know, I regret forking that money out. Thank you all for your suggestions, once I have some time I'll get on it and see if I'm able to fix it.