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Discussion Starter #1
I am getting a PO420 code for converter efficiency below threshold on my 2002 V6 Tacoma with 140k. I have installed new OE Denso plugs, wires and ran several tanks of premium with fuel system cleaner but the code still persists. I have replaced the rear 02 sensor and have checked and the front one is working correctly. It is only 3-4 years old. Both are OE Denso's. Does the fuel trim look ok in the picture? Any helpful suggestions?
 

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Discussion Starter #4
I replaced both sensors 3-4 years ago. I tested both the other day and the front tested good. I went ahead and replaced the rear because the reading was jumping a lot. I ordered two new Walker converters that matched the 50 state California emissions that came on 2002 and newer Tacoma's. The ones I ordered matched the Engine Family # shown on the under hood sticker of my truck #2TYXT03.4FFP. I connected a scangaugeII and set it up to display the air/fuel ratio. It shows 14.5-14.7 at idle. When I blip the throttle it shows rich briefly then settles back around 14.7 while holding a steady throttle.
 

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exhaust leaks will do it. did u clear the codes after replacing sensors or cats............get a piece of hose and listen for leaks. egr not working or vac modulator dirty have all given me the same .... running super will never help higher octane burns cooler and If ur like me the only high performance motors that I own benefit from super are my chainsaws unless you have a lambo porche etc
 

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I checked the exhaust system for leaks and repaired one at the flange between the 2nd converter and rear O2 sensor. No others were found. I cleared the MIL and the code came back in 2-3 days. O2 sensors, fuel trim, plugs, air filter, MAF, PCV valve etc. are all good and no other codes are showing up. Everything points to bad cats. I checked the temperature at the front and rear of both while I had someone hold the rpm at 2,500 for 3-4 minutes. I made sure everything was at normal temperatures before doing this. The front and rear temps taken at each cat did not show a higher temperature at the outlets on either one. I checked for exhaust system blockage by using a vacuum gauge. It held a steady 20" at idle and then 22" when holding at a steady 2,500 rpm. The needle did not show any fluctuations. Seems the cats are not lighting off. I hope to get them installed within the next week. Thanks for everyone's suggestions.
 

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Discussion Starter #8
Installed the new cats on 7/21 and so far the MIL has not came back on. I had read that cats do not just go bad. That something causes them too. But my truck has been checked thoroughly and nothing was found wrong. All readings are normal. The inside of the old cats looked good. No blockage was visible when shining a light through both. The cats had not been damaged or had any internal parts falling apart or were loose. So I guess that cats can just go bad.
 

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Can your scan tool report real-time O2 sensor readings? I have an OBD-Link that I use to connect to my phone by bluetooth and I use one of 3 different programs that I have installed on my phone to graph the voltage coming from the sensors. The front sensor should be constantly varying up and down, while the rear one should be relatively flat depending on if you're in the throttle or coasting in gear where the fuel cut occurs (at which point it will drop real low after a few seconds once the system flushes out any remaining chemicals in the exhaust from the system).

Most of the time my P0420's were caused by exhaust leaks around the converter flanges.
 

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Installed the new cats on 7/21 and so far the MIL has not came back on. I had read that cats do not just go bad. That something causes them too. But my truck has been checked thoroughly and nothing was found wrong. All readings are normal. The inside of the old cats looked good. No blockage was visible when shining a light through both. The cats had not been damaged or had any internal parts falling apart or were loose. So I guess that cats can just go bad.
High exhaust temps or unburned fuel can cause cats to go bad. Unburned fuel will sometimes light off inside the cat.
 

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I had the vacuum line that went from the airbox all they way across the front of the engine to the VSV behind the battery tear just under where it meets the airbox. That threw a 420 and 4xx code for evap leak. Replaced that hose with a fuel line and the code resolved itself. Mine is an 02, just something to check.
 
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