Help with P0420 cat / O2 sensor?

I've been getting a nagging P0420, catalyst bank 1 below threashhold.
It comes & goes.
Is bank 1 the driver side? I did measure the temp before and after that cat, the temp was NOT higher on the downstream side. It was cooler.
I also think my truck smells like gas.

I've searched but not found a specific thread for this. Can someone help me with the process to diagnose whether this is an O2 sensor or a cat? I don't have a fancy scantool, just a plug in and Torque app.
Any help is appreciated!

 

Yes. The bank is defined by the 1st cylinder number at the front of the block. Driver side is 1. Passenger side is 2.

Where do you think you are smelling gas?

Are you on the original injectors?

What was the last time you changed the plugs?

Have you ever changed your coils?

Have you ever replaced the MAF?

Any maintenance related to air/fuel/spark I haven't mentioned?

 

I don't know where I'm smelling gas. Upon startup my wife has complained of gas smell before. I wonder if it's just extremely rich on startup?
I bought the truck 30k ago, as far as I know they are the original injectors.
Plugs were changed about 30k ago. I have not changed any coils, nor replaced the MFA. I do have some MAF cleaner I can try.
I haven't personally done any air/fuel maintenance other than running a strong sea foam cleaning through the tank a few months ago.

I could try cleaning the MAF & changing the plugs to look a their condition.

 

Bank 1 sensor 1 or sensor 2? If it's sensor 2, swap the sensor to the other side (bank 2) and see if the error tracks. If it does, the 02 sensor is the issue.

 

Oh, and one test you can do that won't be so quick and easy for you would be swapping your front/rear O2 sensors from bank 1 to bank 2 and see if the problem follows.

If the code follows to bank 2, I'd replace both of the O2 sensors on the now-faulty side. It's always good practice to swap in pairs if one on a bank goes bad.

Unfortunately, I don't think it'll be an easy* test due to your salt road situation.

 

Jinx!

Two other Q's: Which plugs did you buy, and more important, where'd you buy them? Just curious if you got scAmazon'd with fake parts.

Really, the code is more about your oxygen sensors and what they're detecting. Basically, it believes the pre-cat O2 sensor isn't detecting enough of a difference once that exhaust hits the post-cat O2 sensor. i.e. it thinks your bank 1 cat isn't working correctly. However, if your O2 sensors weren't working correctly, that could be it. I just don't see it as likely because of the added "smell fuel" symptom.

 

On my truck, that driver's O2 sensor before the cat somehow had all of it's wires pulled out. CEL was for the O2 sensor (forget the exact code) and it coincided with PO420. Replaced the sensor but I kept getting PO420 codes that would return pretty much as soon as I cleared the code. That led me to think that the cat was toast too. Do you notice that the truck is a little bit louder, especially around the driver's floorboard? I definitely did with my cat.

Anyways, I replaced that cat with a Magnaflow direct fit that's CARB compliant. The PO420 code stopped appearing as soon as the truck was reset. No issues over the last four years and it's passed three smog tests.

Part number 458017

https://stillen.com/products/2003-2004-toyota-tundra-4-7-catalytic-converter-50-state-legal

 

Thanks for the replies so far. I don't know what spark plugs were used the but the guy I bought the truck from did things right. He used Toyota parts on everything so I'm betting he didn't cheap out on the plugs.
The P0420 code is just "catalyist system efficiency below threshold (bank 1)". This doesn't really indicate upstream or downstream O2 sensor.

Isn't there an upstream (before cat) and downstream (after cat)? IE, are there 4 o2 sensors on the truck?

Should I swap both up & downstream left to right to see if the code follow?
If so, how do you then narrow it down to the upstream or downstream o2 sensor?

 

You need a better reader if you’re not getting a reading on which 02 sensor; 1 = upstream, 2 = downstream.

Autozone, Riley’s or Advanced should be able to read it for you free of charge.

Edit: Now you have me 2nd guessing whether there’s another code that goes with it. I’ll check my truck and get back to you.

 

You narrow it down by the code being given. P0420 is the OBD telling you specifically that the O2 sensor downstream of the cat is sending a signal outside the allowed parameters. As others have said, swap your drivers and passengers side downstream O2 sensors. Clear the codes with a scan tool. Drive the truck. If CEL returns and code P0420 returns, this tells you that your code is not caused by a failing O2 sensor. If the code becomes a P0430, passenger cat underperforming, you have isolated the O2 sensor is faulty since the offending code switched from drivers side to passengers side. No need to touch upstream O2 sensors for a 0420 or 0430 code.

 

gotcha, thanks so much! I must have missed the "downstream" o2 sensor swap in earlier replies. Born in 1981 I think I'm technically a millennial?? So I don't read details & need things explained in baby talk.

 

The upstream sensor does not monitor anything the cat is doing. Upstream sensor only monitors what the exhaust gasses look like before they enter the cat. If the exhaust gasses are not scrubbed in the cat enough to change the oxygen and NOx compounds, than the readings of the downstream O2 sensor are too similar to the upstream sensor and the OBD says "your cat is not working" and throws the code.

 

You're in the right forum for sure!

 

How many miles on the Magnaflow cats so far?

 

40k on the Magnaflow so far, the other one is still factory at 170k.

 

It technically does, in a way, to my understanding.

ECM measures what the pre-cat/upstream senses for combustion mixture, after exhaust gasses exit the block. It then compares that to the what the post-cat/downstream O2 sensor shows for combustion mixture after exhaust gasses pass thru the cat. It can use the delta between the upstream and downstream numbers to gauge the efficiency of the cat. If either sensor was slightly off, it could trigger an inefficiency code, not because the cat was toast, but because the delta between sensors doesn't hit the right range.

This is the only reason I'm saying ... this could be that the upstream sensor and downstream are doing something goofy, or either one acting weird. If it was the upstream, I would expect the ECM to try to adjust the fuel/air if the pre-cat/upstream sensor was reading "off" from reality though (which could potentially cause more gas to be injected, leading to the smell, but I'd expect misfires).

Anyway, I'm open to being wrong, but this is ultimately what I've always understood about the O2/CAT/ECM relationship.

 

We are kinda saying the same thing. And I'm not a cat expert, but the other threads I've read make it seem the upstream monitors exhaust gasses and compares its reading to other upstream sensors, mainly the MAF. Basically everything upstream is doing its best to maintain a 14.7:1 air to fuel ratio. The OBD of course has to monitor a bunch of inputs like air temp, engine temp, throttle position, etc, etc, to tell the injectors how much fuel to squirt, when to fire the spark plug all in the hope of burning a 14.7:1 mixture to minimize the unburned gasoline molecules in the exhaust. My feeling is that if the upstream 02 sensor was faulty, it would also cause OBD to throw codes for fuel mixture and other upstream issues because now the OBD, although properly burning a 14.7:1 mixture, is now being tricked by the faulty upstream 02 to think the mixture is wrong. Since OP only has a 0420 code, I'm thinking upstream 02 sensors are pretty low on the list of culprits.

But I do see you point that a malfunctioning upstream could be tricking OBD into thinking the cat is below threshold.

 

Thanks. I've been fighting an 0430 code for about two years now and was thinking about Magnaflow if I cant get through emissions in the spring.

 

@danmurphymn personally, I'd replace both down stream sensors with new denso ones bought at rockauto only. That's just me though.

There are too many counterfeit parts floating around ebay, and Amazon.

Do you have a multimeter or can borrow one? Do you know how to use one?
Here is the 2UZ-FE oxygen sensor testing using an ohmeter. It shows how to test the upstream (sensor 1) and the downstream (sensor 2.)

 

Magnaflow's cats are one of very few brands that the ECMs in our trucks seem to be OK with...

 

That's good to know! I do have a multimeter & know how to use one. Finding time & desire is the problem...
I've been completely geeking out on my truck bed camping build lately. I really hate to take away from that for mechanical stuff haha.
I'll dig into this once I can find the time & desire. Hopefully this weekend.
This diagram is super helpful, thanks a ton!

 

Here is a link to the entire Field Service Manual. If you're on mobile, you can download it, and then search for key words. That Green highlight is how I found that so quickly. I searched for "Heated Oxygen Sensors"

http://toyota.aitnet.org/Toyota/Tundra/2000-2003_Tundra_FSM.pdf

 

Awesome! Is there a separate one for 04 & newer?

 

Which spark plugs do you recommend?

 

Your engine/drive train is the exact same as 2000-2003. The only difference would be the body... yours being a double cab.

2004 was the first year of the DC, and the last year for non-VVTI engines

 

'04 is, like, that mystery year where there is no FSM readily available in digital form, which is weird to me. I've hunted for it a few times now. The '00-'03 is supposedly interchangeable. The '05-'06 is over here.

Per your '04 Owners Manual (who the fuck reads those?!) this is what you should be using for plugs, where 2UZ is V8, 5VZ is V6. Though I don't think plugs are your problem, never hurts to pull one and check (takes 3 minutes), and DO NOT buy on scAmazon!:

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I've had a recurring P0420 code. catalyst system efficiency of your car is below threshold (Bank 1)
Now the light stays on. I just decided to try to swap bank 1 to bank 2 to see if the code follows, but the downstream on bank 2 has a short pigtale. Bank 1 has a long pigtale, so swapping won't be an option.
Are there any other ideas on how to diagnose this code?
Bank 1 downstream only had 1 (of the 2) nuts holding it on. Would that cause the code?

 

Which engine do you have, again? It doesn't say in your truck info. I assume V8, but that assumption has gotten me into trouble in the past.

 

Anyway, assuming it is v8, I looked it up and you're right. With your 2004 specifically, downstream left (b1?) sensor is Denso p/n 234-4161 while downstream right (b2?) sensor is Densor p/n 234-4162. They look the same to me, one has a longer pigtail. Upstream are matched, Denso p/n 234-4169. I always get my left/right confused hence the question marks, if you have questions about which side is left or right, driver is bank 1, passenger is bank 2, and if you look at the splash aprons in front of/behind the struts, they're clearly stamped out with R or RR for right side, L or LL for left side, which is ultimately how I figure it out.

RockAuto current pricing, before 5% discount:

• 234-4161: $50 (link)
  
• 234-4162: $48 (link)
  
• 234-4169: $40 (link)

I ask you: Seeing that pricing, If you know the sensors haven't been changed in the last 75k-100k miles, knowing my first OEM/factory sensor failed at 13yrs/66k miles, do you feel like maybe you've gotten your money's worth, and does the cost to replace both sensors on the impacted bank really seem outlandish in the bigger picture?

If yes, it seems too spendy, I guess I'd swap the upstream sensors and see if the code follows, just to rule them out. I know it can be a knucklebuster, but ... if you're looking for no-wallet diagnostic steps that could reveal if it's an O2 sensor, and identify whether it's upstream or downstream, that'd be one step I would take if cash strapped. You may also be able to find some kind of harness extension that'll enable you to do the swap. But if you're taking them off anyway, and gonna potentially bust knuckles ... you're only looking at about $100 per side. That's about 5 yards mowed in today's dollar.

 

Quick update - I ordered both O2 sensors from Rock Auto. Bank 1 & Bank 2.
I replaced Bank 1 & the code is gone.
I love it when things are this simple! Thanks so much !