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Discussion Starter · #1 ·
Hello, I’m hopeful someone can help me troubleshoot these catalytic converter codes so I can try to avoid throwing tons of money at the vehicle and not solving the problem. The vehicle has about 234k miles and the codes are for insufficient catalyst efficiency on each bank.

These first showed up after alternator failure mid family road trip. I should have pulled over and had the car towed but my wife did a quick Google search for the nearest shop and I chose to limp the car in about 2 or so miles. It was running really rough and backfiring so I was thinking the O2 sensors got fouled up and were the reason I was getting codes now. I went ahead and ordered all four sensors, but before installation I found a video on how to check your sensors with a multimeter. The old sensors get resistance readings (no “OL”/ open loop), so I’m not convinced I have bad sensors.
Reference video:

One other video I found was to put an infrared thermometer on the exhaust before and after the cat. If the cat is working properly, the exhaust pipe should be cooler upstream of the cat. I’m thinking I will buy a thermometer and check what I get.
Reference video:

In the meantime, I was wondering if anyone had been down this road before and could share what worked. hopefully the cats aren’t ruined because replacement OEM cats are super expensive!
 

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^^^ +1. Anytime I've had problems with these codes on my sequoia it's always been resolved by swapping the downstream sensors. But I never had a problem with O2 sensors that caused rough running or backfire. Usually just a code that sets the CEL. Backfire sounds like something else like timing. I don't see how alternator failure would cause that either but I've never dealt with that so who knows...

Do you still have the rough running and backfire?
 

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2000 Tundra AC 4x4, currently 206K
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On my 2000 Tundra 4.7 one of those codes popped up. I hooked up a shop vac onto the exhaust (on blow aka reverse) and found a missing weld had caused a small leak near the downstream O2. Fixing that cleared up the code. Easy fix...for what its worth.
 

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Discussion Starter · #5 ·
^^^ +1. Anytime I've had problems with these codes on my sequoia it's always been resolved by swapping the downstream sensors. But I never had a problem with O2 sensors that caused rough running or backfire. Usually just a code that sets the CEL. Backfire sounds like something else like timing. I don't see how alternator failure would cause that either but I've never dealt with that so who knows...

Do you still have the rough running and backfire?
Nope, no more backfiring. My alternator was barely giving the car enough power to make spark so unburned fuel was being ignited.

I did not have codes until after the alternator was fixed and the car was back to running well.
 

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Hello, I’m hopeful someone can help me troubleshoot these catalytic converter codes so I can try to avoid throwing tons of money at the vehicle and not solving the problem. The vehicle has about 234k miles and the codes are for insufficient catalyst efficiency on each bank.

These first showed up after alternator failure mid family road trip. I should have pulled over and had the car towed but my wife did a quick Google search for the nearest shop and I chose to limp the car in about 2 or so miles. It was running really rough and backfiring so I was thinking the O2 sensors got fouled up and were the reason I was getting codes now. I went ahead and ordered all four sensors, but before installation I found a video on how to check your sensors with a multimeter. The old sensors get resistance readings (no “OL”/ open loop), so I’m not convinced I have bad sensors.
Reference video:

One other video I found was to put an infrared thermometer on the exhaust before and after the cat. If the cat is working properly, the exhaust pipe should be cooler upstream of the cat. I’m thinking I will buy a thermometer and check what I get.
Reference video:

In the meantime, I was wondering if anyone had been down this road before and could share what worked. hopefully the cats aren’t ruined because replacement OEM cats are super expensive!
Ok this is a bit hard to follow, sorry.

My ‘06 4.7 Tundra with 119,000 miles had the P0158 code, right bank 2 downstream sensor 2, all 4 were original. Changed it with OEM. Cleared the light with my reader, truck ran for 50 miles then P0156 appeared Bank 2 Sensor 2. That’s the sensor I changed!

Then I checked for exhaust leaks, cleaned the MAF sensor, checked all 4 sensors with multimeter both ohms and voltage with mixed results. Swapped both downstream sensor 2s to see if code swapped also, cleared code and drove 3 miles got same P0156, still an issue with B2S2.

So I swapped both upstream sensor 1s and the code changed to P0136 Bank 1 Sensor 2, code followed the swap!
That told me the computer was flagging the wrong sensor (I’d heard of that) so to verify (and have for future) I ordered a $300 code scanner that shows live data.
Since the scanner is only reading what the computer spits out via codes, the readings indicated B1S2 was lazy/unreliable compared to B2S2. Recall that I replaced B2S2 still had P0156 & swapped it with B1S2 and code stayed the same…hum.

So I swapped B1S1 with B2S1 and code switched to P0031 Bank 1 Sensor 1! So must be bad B2S1 which I’d swapped with B1S1. Have I lost you yet?

Bottom line is the computer was throwing the wrong code. Problem was bad air/fuel ratio sensor 1 on bank 2. Since I’d swapped sensor 1s I replaced B1S1 with OEM and no more problems, for now.

Nothing was wrong with my original B2S2 that I’d swapped to bank 1 with no change, only when I swapped both upstream sensors did I get a difference.
And my scanner was reading the same wrong sensor that the computer was showing, only by swapping sensors did I discover the computer lying!

I’m tempted and probably will replace the other two sensors so all are fresh, good for another 16 years.

My take-away is buy a scanner with live data and don’t believe everything you see😆!
 

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Discussion Starter · #7 ·
Ok this is a bit hard to follow, sorry.

My ‘06 4.7 Tundra with 119,000 miles had the P0158 code, right bank 2 downstream sensor 2, all 4 were original. Changed it with OEM. Cleared the light with my reader, truck ran for 50 miles then P0156 appeared Bank 2 Sensor 2. That’s the sensor I changed!

Then I checked for exhaust leaks, cleaned the MAF sensor, checked all 4 sensors with multimeter both ohms and voltage with mixed results. Swapped both downstream sensor 2s to see if code swapped also, cleared code and drove 3 miles got same P0156, still an issue with B2S2.

So I swapped both upstream sensor 1s and the code changed to P0136 Bank 1 Sensor 2, code followed the swap!
That told me the computer was flagging the wrong sensor (I’d heard of that) so to verify (and have for future) I ordered a $300 code scanner that shows live data.
Since the scanner is only reading what the computer spits out via codes, the readings indicated B1S2 was lazy/unreliable compared to B2S2. Recall that I replaced B2S2 still had P0156 & swapped it with B1S2 and code stayed the same…hum.

So I swapped B1S1 with B2S1 and code switched to P0031 Bank 1 Sensor 1! So must be bad B2S1 which I’d swapped with B1S1. Have I lost you yet?

Bottom line is the computer was throwing the wrong code. Problem was bad air/fuel ratio sensor 1 on bank 2. Since I’d swapped sensor 1s I replaced B1S1 with OEM and no more problems, for now.

Nothing was wrong with my original B2S2 that I’d swapped to bank 1 with no change, only when I swapped both upstream sensors did I get a difference.
And my scanner was reading the same wrong sensor that the computer was showing, only by swapping sensors did I discover the computer lying!

I’m tempted and probably will replace the other two sensors so all are fresh, good for another 16 years.

My take-away is buy a scanner with live data and don’t believe everything you see😆!
Ok, so I don’t have the same exact codes, but I do have a scanner. I played around with the live data and some diagnostic tools and here is what I got.

My sensors are recording data, I just need to try and figure out if I can trust it. The cat temps seem to be higher on the upstream sensors which from what I can tell it should be the opposite. This is both at idle at operating temp and when the engine is held at 3k rpm.

Next are the catalyst tests which both narrowly fail based on the defined lower limit. Not sure what is being measured for these two tests.

Finally I have some live data values for O2 sensor voltage and short term/ long term fuel trim. One set of photos is for idle and the other for 3k rpm. I noticed the fuel trim values would tweak and my rpm would drop slightly as the system is adjusting the air/fuel ratio on demand. This would happen every 30 seconds or so. But I’m not sure what values are normal here.
 

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I tried replacing one at a time on my 2004 Sequoia and codes stayed, had to replace all 4, and haven't had any issues for over 4k miles. I see the values seem to read right so I would try to run some non-ethanol gas and maybe some cat-a-clean or lucas fuel treatment to clean the 02 sensors and cats if either is dirty. If worst comes to worst I'd do the 02 sensors as with it being 2006 with 234k [My 02 sensors went out at 230k for reference] I'd replace them with OEM densos and go from there.
 

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Discussion Starter · #9 ·
“Bad News Bears”. This is with the engine warmed up and holding the revs at about 3k rpm. The exhaust is measuring considerably hotter before the cats on both sides. . .
I must have melted my cats from driving it on that bad alternator with all the exhaust backfire. A tow truck would have been so much cheaper. Hindsight is 20/20. . .
 

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Discussion Starter · #12 ·
What are typical numbers for those temp measurements?
I’m not sure, but the main takeaway is the honeycomb material in the cats gets glowing red hot to burn off pollution so the exhaust exiting the cat should be hotter than exhaust going in when working properly.

I have an appointment scheduled with my local muffler shop. They agree with my diagnosis and will weld a reliable set of universal cats in for under $700. Just one OEM assembly for one side is more than double that and out of stock. They recommended new OEM downstream O2 sensors to go in at the same time since those thread into the cats and got torched too. I already have those in hand.
 

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Discussion Starter · #13 ·
Alright, so thought I would give an update. The local muffler shop welded in the new universal cats this past Monday (5-16-22) and I had them install the two new OEM (DENSO) downstream sensors into the new cats. The check engine light came back on just today, maybe only 40 or so miles after the install with the same two codes.

The only other things I know to try are installing new upstream sensors and I’ve heard you can try cleaning the MAF sensor in the air intake. Any other ideas? Maybe the cats that were installed are defective?
 
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