Help with P0420 cat / O2 sensor?

I put a B1S2 sensor in in both pipes. The one in B2 was known to be bad. The one in B1 was questionable. Took the truck for a drive ended up with 0430, 0136, 0156 codes. Bought a B1S2 and returned the original B2S2 to original position. Cleared code but 0430 came back after 80 miles. I was able to get a drive cycle and my inspection done before the 0420 code so I'm happy. For giggles I put can of sea foam in the gas tank. Now just waiting to see what codes, if any, return.

On a side note. I've been using a dongle with torque pro and a graphing app that allows me to watch sensor voltages. What a difference it made in diagnosing the problem. Best part, it's only about $60 for the entire set up.

 

Did you pull the negative battery cable for ~10min after install to force ECU to re-learn? I can see how you may throw the other codes, if not, ECU is judging efficiency based on delta [upstream<=>downstream], I wouldn't expect a any significant break-in period, but ...

For same reason, if everything is torqued down proper, I'd be skeptical of the P0430 (or 0420, whichever was accurate). I'd give it a while and clear the code a few times, let the ECU re-acclimate to the new O2 sensors. And if you didn't unhook the neg batt cable, go pop it off for 10-15 mins, and don't be surprised if it runs like shit for the first dozen or three miles after.

 

After the 0420 code I didn't pull battery cables. I was pressed for time trying to get inspection done. All monitor were cleared with exception of evap system so I got inspected without issues. I tried removing B1S1 but no way that sensor was coming out. Plus the O2 sensor socket I bought at Napa was real sloppy on on the sensor. Didn't want to ruin the hex on the sensor.

 

Ok, got confused earlier after seeing both 420 & 430.

If it's B1S1 you couldn't replace, 0420 makes sense: ECU is taking data* from old, tired sensor B1S1, then calculating its delta based on comparison from new, fresh B1S2. There's about a dozen ways that could go awry, and it's one of the common cases that lends to 'replace both at same time'.

But I also get it - the upstreams can be a freaking PITA to pull. Best advice I can give is to keep hitting it with penetrant after decently long drives, and consider dropping a little coin on a good, hardened sensor wrench. OR see if your LAPS has a loaner tool you can check out, as many do.

 

That's exactly what I do. Quench it with penetrating oil. Always has worked. Just didn't want to take a chance with eminent NYS inspection.

 

Confused on what scanner read. P0420 Catalyst system Efficienty Below Threshold Bank 1. Is that Upstream before the converter or Downstream after the converter?

 

That code translated: The ECU thinks the catalytic converter is worn out, because the readings from the upstream and downstream O2 sensor equal a delta/difference indicating the cat isn't filtering enough stuff.

The truth is, the front O2 sensor is older, and likely not correctly identifying the exhaust composition as it leaves the manifold, so when the ECU produces its delta based on what the new, fresh downstream O2 sensor is putting out, it's naturally outside the threshold of what the ECU wants to see. Has nothing to do with the cat at all.

The P0420 and P0430 codes are commonly seen when you have one old and one new O2 sensor, or when one of two O2 sensors is failing, or or people who don't listen and choose to use aftermarket O2 sensors, or when someone chooses to only replace a single faulty O2 sensor on a single bank, vs. replacing in pairs. It's also a common code to throw on these trucks for people who choose aftermarket cats, aside from 2-3 brands forum members have identified as 'never throws codes'.

Truth of the matter is, if you're using all OEM sensors and haven't touched your cats, and you haven't been running with rich/lean codes or other codes that would affect air/fuel ratio, if you get the P0420 or 30 codes, you'd be wiser to replace your O2 sensors for that specific bank rather than buy new cats. Maybe if you're up in the 250k-350k mile range and you see that code, it's feasible your cats could be the issue. But I'll (personally) always suspect the O2 sensors before the cats, unless there's a valid reason not to (i.e. I just replaced both pairs of sensors on that bank, and I know 100% the source I purchased isn't scAmazon, fleaBay, WallyWorld, or some other source that's known for selling knockoff parts)

 

I had replaced the bank 1 & 2 before the cat and bank 2 after the cat between 2012 & 2015. I just turned 260,000 miles on it now. I had replaced all three with Denso's from Rock Auto.

So is Efficienty Below Threshold Bank 1 before or after the Cat?

 

Can't stress enough, you really should replace both O2 (upstream and downstream) any time one sensor goes bad on a bank. And your case is ultimately why I am telling you this.

P0420 = Bank 1 (driver side, if 4.7L) has a bad delta between the upstream/downstream sensor
P0430 = Bank 2 (passenger side, if 47.L) has a bad delta between the upstream/downstream sensor

It sounds like you've replaced:
Bank 2 Sensor 1 (B2S1) --> Passenger side upstream (before cat)
Bank 2 Sensor 2 (B2S2) --> Passenger side downstream (after cat)
Bank 1 Sensor 1 (B1S1) --> Driver side upstream (before cat)

If you're throwing P0420, that's telling you one of three things, given what you already replaced:

1. Your driver side downstream O2 (B1S2) is NOT functioning correctly, and is feeding the ECU bad data which makes it think your driver side cat efficiency is low.
2. Your driver side cat is actually clogged and not doing its job.
3. You got a DOA part from Denso (B1S1), so basically #1 but the "new" sensor is at fault.

Look, if #2 of those was true, I would've expected the ECU to have noticed it prior to you swapping out B1S1. But even if it was, it could just be because the downstream sensor (B1S2) has been bad this whole time and the actual sensor you needed to replace.

Reality is, #3 likely is not true, because QA/QC from Denso is reputably good.

Throwing on a new B1S2 sensor to isolate whether #1 is the problem is the cheapest possible solution. Driver side upstream sensors for your truck are the same Denso p/n for both sides but if you're 4WD, your rear/downstream/after-cat passenger side sensors are different p/n. But whichever it may be, a new O2 sensor is only $50-60 max, versus a cat which will cost you a fortune. You wouldn't be the first person to find that simply replacing your O2 sensor solves P0420 for you, knowing what you just told us about not replacing both sensors at the same time on each bank. I don't think it's a coincidence the bank with the oddball old sensor is throwing codes.

Just make sure you buy Denso O2 from a legit source. Downstream left for 2WD and 4WD should be 234˗4161.

 

Thank you for all that information. Will get sensors and replace.

 

The two I replaced up stream were 234-4169 back in 2012 & 2015. Can’t find the downstream part #.

 

You have the V8, right? Is it 4WD or 2WD?

 

Either way, here you go - pick the correct drivetrain type in the dark grey header to see the correct part numbers out of these two, either the grey header with 4WD or the grey header with 2WD. You'll see 4WD trucks have different downstream sensors, probably the same sensor, but the pigtail is shorter on one.
This is why we ask everyone to include the engine side and drivetrain in your truck info (blue box underneath your screen name on the left.

 

Greatly appreciate it. Thank you very much.

 

I've recently had the same issue on my 2015 4WD V8 Tundra, it threw the P0420 code and I figured it was a faulty O2 sensor because the truck only has 40,000 miles on it. I swapped the B1S2 and B2S2 O2 sensors and still got the P0420 code pretty quickly after resuming normal driving activity.

I know there are a number of reasons that the code could be triggered so I borrowed a friend's OBD2 code reader and found more reason to scratch my head. Nothing is misfiring, O2 sensor voltages are normal, at idle the catalyst temperature sensor 1's are 340 degrees and the 2's are 183 degrees. Am I missing something here? Very surprised to see this code at 40,000 miles for a Toyota engine. I am now convincing myself it smells gassier than normal and starting to lose my mind. Any help would be much appreciated.

 

"Smells gassier", maybe you do have a clogged cat?

But if your 2015 is the same as the 2000-2006 we're discussing in this forum, maybe you are missing something? P0420 is Bank 1 cat efficiency. You said you replaced the rear O2 sensors (B1S2, B2S2). You should've probably should've replaced B1S1, B1S2 sensors. I'd be suspicious of that pre-cat O2 on bank 1 (B1S1). Remember, the pre-cat sensor is what's scanning the combustion mixture straight out of the cat, which lets the ECU know if it's burning rich or lean. If it's fucked, it lends to reason you may be running rich. That condition will clog your cat. My next step would be replacing B1S1.

But take that with a grain of salt. Again, you're in the subforum for 2000-2006 Tundras. Our engines are different (and more reliable) than what's in your 2015

 

Sorry, I wasn't clear on the first post. I didn't replace the B1S2 and B2S2 sensors with new sensors, I just switched them to see if the code followed a sensor and determine if it was just a bad O2 sensor, which was not the case. Are you suggesting I do the same with the B1S1 and B2S1 O2 sensors?

It's my understanding that if I were to replace one O2 sensor, then I should replace both S1 and S2 on whatever side needed the replacement.

I know this is 1st Gen subforum but it has way more information and help than anything I can find for a 2nd Gen. Thanks for the help!

 

Not a problem at all.

Let's think about that logically. If you're consistently throwing a P0420, even with only 40k miles, the ECU thinks your bank 1 cat efficiency is low. There are only a few logical reasons why that could be the case.

• At least one of your O2 sensors are feeding the ECU bad info, so when it compares the upstream and downstream sensor values, it thinks your cat isn't properly filtering
• Your cat is actually clogged, and thus it cannot filter properly, hence why the delta between upstream and downstream sensors is beyond acceptable threshold to throw that code
• Your ECU is borked, and not interpreting things properly (which is a stretch - you'd likely notice other stuff wrong)
• There's another issue at play, like, maybe a ground for your O2 sensor circuit is corroded, wire melted/chewed, or similar condition not allowing a sensor to behave right (I find this unlikely too, but ... worth mentioning)

You've shared two details:

1. I swear I smell fuel and it's getting worse
2. I've got P0420 code

Given the upstream sensor is what tells the ECU your combustion mixture, and adjusts air/fuel ratio based on that input, it serves to logic your upstream O2 could be going bad. I say that because it sounds like your ECU may be getting invalid a/f data from exhaust out of the manifold and adjusting up fuel accordingly, which is lending to unburnt fuel in your exhaust, which you're then smelling. And that WILL clog your cat.

If I had no good quality reader that let me inspect what the O2 sensors were doing in realtime, and I were in your position, I would consider replacing the upstream sensor, clear the codes, and see if it resolves. I assume your sensors on the 2015 may be as cheap as ours on 1st gen (many from Denso are $50). ONLY USE DENSO PARTS. See if it resolves the problem. Yes, it's technically "throwing parts at the problem" because you're not scanning. But based on those two points ... it's logical to get to this deduction.

 

All of that makes perfect sense to me and just a side note, I really can't say thank you enough for your time and knowledge to help me troubleshoot. One reason why I got a Toyota in the first place was to learn how to work on it and your collaboration means a lot to me.

With that said, would it be a good idea to switch the current B1S1 and B2S1 O2 sensors, clear the code, and then see if it follows to the passenger side to determine if it is a sensor issue before buying a new one?

That way if the code follows to the passenger side, then it's a simple fix to replace the bad sensor. If the code remains for the Bank 1 Driver side, then I've ruled out the sensors being faulty and can continue to troubleshoot without buying an unneeded sensor.

 

Yes, if you look it up at www.densoautoparts.com and find both upstream sensors share the same part number, swapping the upstream sensors left to right and waiting to see if the code follows could be one test; if no joy, and the downstream sensors also match part numbers, you could attempt swapping those to see if the code follows. You're right; I jumped to conclusions on that one, and your option would be the wise one if it's possible, I was running on some assumptions, see 3rd paragraph.

FWIW, O2 sensors aren't infallible. Some owners on here have had their sensors last for 200k miles and 18-20 years. Me? My 1st sensor went out about 3 years ago, at 75k miles and 15 years. It's not unheard of for them to fail within 8-10 years. That's partially why I'm, like, "Yeah, probably bad".

It looks like 2015 still had a 4.6L engine option, but all Limited models came with the 5.7, so ... it appears you just need to determine whether your truck is ethanol-blend "FLEX fuel" ready or not. I assume there will be badging somewhere saying "FLEX" on the gas door, maybe? If it's FLEX-ready, your upstream sensors are different (driver = left, passenger = right). If not, they're the same. I assume all vehicles after 2012 are FLEX-ready, which may or may not be wise.

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