P0420 After Valve Cover Gasket Change

Change the valve cover gaskets on the 2006 V6 yesterday. Took for a drive afterwards and got a P0420 catalytic converter code….I reset it once I got back home and drove today, same thing, code reappeared. Not sure if this is just a coincidence or what??? Did not have this issue prior to gasket swap, checked everything to ensure I didn’t leave disconnected, is it possible I could have damaged anything during the work. I googled the sensor locations and from what I see this shouldn’t be an issue.

Any input appreciated in advance.

thanks, Griff04

 

Hey Ralph,

It could be purely coincidence. What's your mileage, and have you ever had an O2 sensor go bad?

If over 70k miles, and you've never replaced a sensor on bank 1 (typically driver's side), it may legitimately just be a bad O2 sensor. I don't normally believe in coincidences, but ... for the record, my '06 has 74k miles on it today, and one of the bank 1 sensors went bad at around 71k miles.

 

Are all your vacuum hoses seated and in their proper locations? No cracks in the hoses?

 

Good points, and worth mentioning. The whole purpose of O2 sensors are to test the combustion mixture BEFORE the cat, then AFTER the cat.

Any extra introduction of outside air before hitting the pre-cat O2, or after the post-cat O2 could cause issues with that reading. So exhaust leaks, vacuum leaks, any of these things could cause extra O2 being read by the sensors. I dunno about the V6, but it's a f'n bear all the damn hoses you've gotta work around on the driver's side of the V8, so @FrenchToasty could be phoning it in here on the cause!

 

Thanks for the input, I’ll take another look tomorrow to see if I may of knocked a vacuum hose off of anything. There is a vacuum hose on the backside of the breather assembly that I had to disconnect when removing…that’s probably a good starting point. Not sure if that has anything to do with that particular code or not.

 

P0420 is a generic way of saying "what I'm seeing at the post-cat O2 is not copacetic with what I'm seeing from the pre-O2 cat"

In plain English, "If I'm going by what the pre-cat O2 sensor is saying, the difference/delta I'm reading from on your post-O2 cat says your cat isn't filtering enough shit out".

So there's a couple possible problems:

• You're introducing bogus data to the ECU, by way of leaking raw oxygen post/pre-cat (manifold leak, vacuum leak, or other external tainting of data)
• One of your O2 sensors if malfunctioning, giving the ECU bad data
• Your cat on that bank finally shit the bed, and is clogged enough to not be doing its job

When this type of code comes up, we all hope it's not the last of those three items. Often times, it's the middle of the three. But in the case of a recent maintenance, it's not unheard of for it to be the 1st of those three.

There obviously could be other explanations, just going with the top three here... If you unhooked any wiring while doing the job, poor electrical connection could be the culprit of course. But ... we're not there, and weren't there, and aren't there, so we can't say for sure.

The general point here is: Rule out that it's any of the three as best you can. Then report back if you figure it out, so others can learn from your issue.

 

Took another look today to see if I may have left something loose…didn’t really find anything abnormal. Reset the code again, twenty miles later the check engine light and code came back.

So exactly what is this code telling me? I understand the Bank 1, driver’s side, but the 1/2 is throwing me off, is this just referencing banks 1 and 2 or bank 1 sensor 2 which is the downstream sensor. How do I determine the upstream sensor vs the downstream sensor from this? Also I read a couple of threads mentioning to only use “Denso” sensors, are they just a brand name…what makes them special??

 

Denso and Aisin are Toyota OEM brands. I know this probably sounds like complete bullshit, but with these trucks, there are certain parts you absolutely don't want to buy anything except Denso or Aisin, or you're putting your life, your wallet, or your sanity at risk. Especially O2 sensors, fuel pumps, coil packs, and unless you like snapped balljoints, never use aftermarket lower balljoints ... I can share dozens of sad pics of Tundras w/front wheels snapped off because someone didn't listen.

You also don't want to buy parts on scAmazon or fleaBay to save $$, because they're flooded with grey market & counterfeit parts, which has burned many of us already. It's really bad with auto parts, factories in China churn out knockoffs all day long, they get shipped to scAmazon warehouses where any store selling on Amazon (doesn't matter the seller) who chooses to use Amazon warehouses for fulfilment will send you items from shipment. scAmazon has no reason to fix this problem - they're not legally liable and it's difficult to prove, but some guys on here have gotten on the horn with NGK, for example, and given lot #s off their part, and been told - "We never made a part with that lot # on it".

The (1/2) on your screen is just telling you either (A) this is page 1 of 2 for this error and you need to tap the red down triangle on the reader to see the 2nd page (2/2) or it's also possible that (B) it's telling you there's two codes, and you're only looking at the first of those 2 codes, and again, you need to tap the red down triangle button to see the other code. You'll want to check that and get back to us.

"P0420" OBD code tells you your ECU thinks your catalytic converter is not filtering properly.

How does it know this?

Like all modern cars, there are sensors all over the engine, sensing volume of incoming air, throttle plate position, whether things inside the engine are spinning (crank, cams), whether the engine is knocking, whether there's proper vacuum in the fuel system, etc. This is how the ECU is able to keep track of everything happening on the vehicle, and ensure it adjusts and fine-tunes itself to run optimally, cleanly, and not have a catastrophic event.

Two of those sensors are on the exhaust. For every catalytic converter in a modern vehicle, you'll find an oxygen/O2 sensor before the catalytic converter (the "upstream" sensor), and an O2 sensor after the catalytic converter (the "downstream" sensor). Why two? The ECU wants to measure what your exhaust looks like before it hits the filtering element of the catalytic converter, then check it again after. If it doesn't sense enough emissions garbage is getting filtered out by the catalytic converter, the ECU will set a code, P0420 if it's on the exhaust for 'bank 1' cylinders (i.e. driver side, cylinders 1/3/5/7), and I think P0430 if it's on the exhaust for 'bank 2' cylinders (i.e. passenger side, cylinders 2/4/6/8).

Is it necessarily true, "I popped a P0420", so my cat isn't working right, needs to be replaced"?

NO. It doesn't.

Thinking to what I just told you above, say your upstream O2 sensor was dying, and sending the ECU bad info about your exhaust gasses before it passes through the catalytic converter. If that were to happen, when the ECU compares the readings from the upstream and downstream O2 sensors, of course it's going to get a whacked out result. If the bad O2 sensor is at least still good enough to produce output that looks semi-legitimate, the ECU could choose to blame your catalytic converter, and throw a P0420 as yours did. If the O2 sensor is clearly dead, or the value is so whacked out that the ECU knows it's too out-of-range to be realistic, it'll throw a code for bad O2 sensor (and it gives a diff't code for upstream sensor, and downstream sensor).

I talked about non-OEM (non-Denso, non-Aisin) parts earlier and the problems they cause. There's been more than one person who came here or other Tundra forums with the same code you have today. The V8 (2UZ-FE) in these 1st Gen Tundras is really sensitive to catalytic converters, just like it is with O2 sensors. There are plenty of people who chose to save money going with the cheaper parts store catalytic converters after getting this code. The part definitely works, but the part doesn't meet or exceed the threshold of filtration Toyota programmed their ECUs to accept, so those guys continue throwing the same code repeatedly, unless they swap out with a known-compliant part. So far, we know the Summit Racing brand and Magnaflow do meet Toyota's threshold. This is info you may need if you replace your cat.

You probably have one of a few things going on, realistically:

• Your upstream O2 sensor is on the way out, fooling the ECU into thinking your cat is bad.
• Your downstream O2 sensor is on the way out, fooling the ECU into thinking your cat is bad.
• Your cat is actually at the end of its life and needs to be replaced.
• Someone already replaced the cat(s) and/or the O2 sensor(s), but use some brand other than Denso or Aisin, and the brand they chose isn't up to snuff by Toyota's standards, which is actually more common than you may believe
• There's another issue causing the O2 sensors to report issues, like, someone used non-OEM brand parts, and had to cut/splice the replacement O2 sensors in, and this part of the emissions system is really sensitive to breaks/interruptions in the circuit - you don't want to cut/crimp/solder/splice anything into your O2 wiring.

Most of us who know better will usually replace both O2 sensors on a single bank if one goes bad. Bank 1 upstream is bad? Replace both sensors on bank 1. Bank 2 downstream is bad? Replace both sensors on bank 2. That's because if one failed on that side, the other is often not far behind. It's better just to bang your knuckles up once, and be done with it for another 100k miles, y'know?

Hopefully this all makes sense.

 

Thank you for your reply..it really makes sense. It didn’t even dawn on me about “page 2”. I’ll take a look at it again tomorrow along with rechecking all the wiring connectors and vac lines, I just don’t want to arbitrarily start replacing parts without being sure first…I’ve been down that road too many times. In regards to using OEM parts I totally agree, there are consumable type parts that can be swapped out but in the long run it’s well worth spending the extra $$$ upfront to prevent headaches further down the road. I’ll reply tomorrow with my findings.

 

Update…well honestly I’ve spent more time in the woods deer hunting vs working on my truck. Last Wednesday, the last day I drove it, I pulled everything associated with the breather assembly just to check all the vac hoses and electrical connector connections. I did find one near the front of the housing assembly, not sure what it is or what it feeds, that was not completely snapped in. Put everything back together and the P0420 code was still there when I started it. Today was the 1st day that I’ve really driven it, went to Walmart and the code was still there, when I got in the to leave and started it up the check engine light was not on. Put the code reader on it when I got home and the code did not come up. I didn’t know these could reset themselves. So in the reality I believe it’s still up in the air on whether or not the loose connector had anything to with it at all. I’m just glad the code cleared. Only time will tell.

Thanks to everyone for their replies. Regards Griff04

 

Thanks for the update. On a lot of OBDII vehicles, the ECU/ECM is running regular checks, long and short term. It's not uncommon to have a code unset/delete once an error is corrected. Not true for all, but true for many. Loose gas cap is a classic.

 

I've had P0420 come and go ever since installing headers 2 years ago. Every couple months it will trigger the check engine $ light which will stay on for a few days and then clear itself, only to return in another 3 to 6 months.

 

Spoke too soon. Code reappeared today. Back to the drawing board.

Griff04

 

Okay back at it again. I don’t want to just start throwing parts at it so I did a little research online. A couple of things I ran across was ohming out the O2 sensors to see if they were good and another article mentioned checking the inlet and outlet temps on the cat converter using an infrared heat gun which stated the outlet temp should be between 150 and 200 higher than the inlet side. My readings today were 450 inlet and 350 outlet….do these numbers sound correct, if I go by the article that I read then the cat converter is bad?

 

It's just so weird to me this started after your VC gasket change.

If it were me, and I had the time versus wanting to spend money, I'd consider swapping both driver bank O2 sensors with the passenger bank O2 sensors.

If the passenger side (bank 2) cat starts popping a code for 'efficiency below threshold', then you know it's an O2 sensor malfunction, and not your cat.

If you clear codes after and it pops another P0420, then there's a solid chance your cat is clogged and needs replacement, or there's a driver-side-only wiring prob.

Of course, I'd check all my grounds, then the wiring to my sensors before the sometimes-knuckle-banging work of pulling O2s, but ...

 

Yes, I’m puzzled also about the code after the VC gasket change. I think I’m going to just purchase 2 new sensors for bank 1…they’ve probably never been changed, I purchased it with 185k and put 15k, they could probably use a change.[/QUOTE]

 

Usually this is a great idea but remember OP has a V6. Though this is a 4.0 I assume it still only has one bank like the 3.4 but I'm not certain.

[/QUOTE]
I think this is a good idea whether your cat is bad/clogged or not.

 

Crap, I keep forgetting he's VVTi V6. Not my engine, so I haven't read a ton on it.

 

We don't get a lot of 4.0s in here. There's only three or 4 that have posted, and stayed around.

 

Did I post in the wrong place, is this a V8 forum only, I thought it was for all 1st Gens??

 

You didn't. Just me Nick reminding me I'm a dumbass to forget you've got a V6, and me lamenting that he's right, I am a dumbass for thinking in V8 vs. V6

But it looks to me like the 1GR-FE has dual exhaust manifolds, doesn't it, @Griff04 ?

I checked the FSM for S&G, it points us to diagnostic section DI-228 which has the following hint where the code ending '20' is bank 1, code ending '30' is bank 2:

​

It basically reaffirms what I was saying: You either have an exhaust leak, you have a bad oxygen sensor, or your cat is clogged. If dual exhaust, a no-cost-but-time-and-blood test you could run would be swapping the O2 sensors over and seeing if the code follows.

But it can't hurt to check for exhaust leaks too.

 

Just throwing this out there but how about torque specs on the valve cover?? Don’t know if OP mentioned if he had torqued the bolts or not. Would that even cause an issue as well?

 

I wouldn’t think the valve covers would throw this code…actually I could have taken some of the bolts out my hand they were so loose. I did torque to spec though.

 

Yup, that's pretty typical. Gasket dries out/shrinks with age, takes away the compression. Some folks find just a re-torque will stop any leaks that've come up.

 

All 1st gens are welcome here. He was just saying we haven't seen a lot with the 4.0. Fairly rare at this point.

 

Denso sensors came in today…gonna try to get them changed out tomorrow after work. We will see how it goes.

 

Got off late, only had a chance to replace the downstream sensor and reset the code, will drive it tomorrow and see what happens. I’m attaching a pic of the old one, doesn’t look that bad to me but really don’t know what a good used one should look like.

Also ohmed out the heater wires on the old one, the new ohmed 15.3 and the old was 12.5, not sure if that means anything, I’m thinking if it was bad there wouldn’t be any resistance at all??

I will pose this question though…if the upstream sensor was bad wouldn’t the truck be running bad since it controls the fuel mixture…just asking, don’t really know.

thanks, Griff04

 

Only thing that looks suspect to me is the greasy spot. Hoping it is from a finger or you tagged the bung when removing it. Other than that, good ones and bad ones are hard to tell from just a "look." As far as the heater, that difference is negligible. I actually have a bad heater on one of mine, that has been in the truck for like the last 7 years or so. I scan it every so often just to make sure its still just the heater portion. Ill replace it when it finally fails hard core.

 

To really ohm out an O2 you have to heat them up while reading the resistance. I do t remember the numbers you're looking for, but they'll change when they heat up.

 

Either of those readings are good as far as the Heater Circuit of our O2 sensors according to FSM.




If one wished to bench test the Sensor Circuit of our O2's they could apply flame of Propane torch to sensor. Once the sensor has reached a certain temp the sensor will start producing a DC voltage which should approach 1 vdc. It's the reading of 0 to 1 vdc that is a feed back to ECU that says the oxygen content is either Lean(approx "0" vdc) or Rich(approx "1" vdc)