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‘04 Tundra 4.7 186k miles getting P0420 Catalyst System Efficiency below threshold bank 1 CEL pretty steady now. Initially it was intermittent for months. I haven’t done O2 sensors in years/many miles. Could this be an O2 sensor or bad CAT? What are the implications if it’s a bad cat? I need to tow heavy this coming weekend and am afraid of it running lean. Thanks.
 

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‘04 Tundra 4.7 186k miles getting P0420 Catalyst System Efficiency below threshold bank 1 CEL pretty steady now. Initially it was intermittent for months. I haven’t done O2 sensors in years/many miles. Could this be an O2 sensor or bad CAT? What are the implications if it’s a bad cat? I need to tow heavy this coming weekend and am afraid of it running lean. Thanks.
Do the least expensive options first.
Larry
 

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Discussion Starter #3
So you’re saying it could be an O2 sensor rather than a cat?
 

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Discussion Starter #4
When I’ve gotten that code in the past for an O2 sensor it didn’t say catalyst threshold
 

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when i've gotten that code on my sequoia it's always been resolved by replacing the downstream o2 sensor or by fixing the rusted out bung on the exhaust pipe. at 265k, i've replaced my downstream sensors a few times due to that code. my cats are still fine. don't get a cheap sensor. get whatever the oem brand is. forget if that's denso or whatever but don't get a cheap one.
 

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Discussion Starter #6
Hmmm. I was wondering if an exhaust leak could cause it. I was also seeing posts where people claim it’s usually the upside. At least I know I can start with a thorough visual inspection then try my O2 sensors. I’ve always used Denso in the past. Is there something I can spray around the O2 bungs that won’t hurt the sensors or cat while I’m scanning to see if it is a leak?
 

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I've seen youtube people spray soapy water on startup before everything is hot to check for leaks. Never really did it myself. Mine were so bad you could hear the leaks (mechanics stethoscopes are good for this if the leaks are small). And you could easily see the nuts/studs/welds rusting away so badly that the sensor wasn't held firmly. not hard to diagnose leaks when they look like that.
 

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also, if i suspect downstream sensors, i swap them side to side to see if the code follows the sensor. the downstream are easy to change in just a few minutes. i would also try to log with the 02 sensor waveforms look like with a scanner before spending money on the upstream ones.
 

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‘04 Tundra 4.7 186k miles getting P0420 Catalyst System Efficiency below threshold bank 1 CEL pretty steady now. Initially it was intermittent for months. I haven’t done O2 sensors in years/many miles. Could this be an O2 sensor or bad CAT? What are the implications if it’s a bad cat? I need to tow heavy this coming weekend and am afraid of it running lean. Thanks.
Same message for me after having done all four O2 sensors 18 mos ago. Scared it was the cats as well thought I would just hit the Mass Airflow sensor with Mass Airflow spray cleaner thinking maybe it’s due and it worked! Code went away and been gone for 2 mos. 302k on mine w original cats
 

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‘04 Tundra 4.7 186k miles getting P0420 Catalyst System Efficiency below threshold bank 1 CEL pretty steady now. Initially it was intermittent for months. I haven’t done O2 sensors in years/many miles. Could this be an O2 sensor or bad CAT? What are the implications if it’s a bad cat? I need to tow heavy this coming weekend and am afraid of it running lean. Thanks.
I have a 2004 Tundra also with 194K. Had the same error code. Replaced all 4 O2 sensors about 2 years ago and no problems since. Dealer of course wanted to replace the CAT.
 

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Discussion Starter #13
As you may have seen on my post for removing downstream sensors it was triggered by rusted off mounting nuts and therefore a leaking gasket. New stainless nuts and new gaskets fixed the problem. Cats are fine.
 
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