Hi, so yes the P0420. The info:

06’ V8, 188k. Did the down stream about a year ago. Light went off for a while. Never done the upstream.

The problem:

Every time I start the vehicle and drive for 10-15 mins (on the highway most times) the light comes on. I clear it, while driving (I know) and it’s always P0420.

So, here’s what I’m thinking. If I do the downstream spacer idea and it doesn’t work, should I then try replacing the upstream? Because the spacer should fix the light if it’s a bad cat, but not fix light if it’s a sensor - I have that right?

Thanks I’m advance

So got upstream replaced today. First time out no light. Yay. Second time on highway came back on.

So it comes on only when I get on the highway for the first 2-3 mins. Never comes on any other time. If I clear it while on the highway, it doesn’t come back while I’m driving. So weird.

Any thoughts on this?? Could this mean there’s another cause behind this - MAF, speed sensor, I dunno.

Please someone with some ideas!

Make sure you have no exhaust leaks especially where the y-pipe attaches. I have a similar issue but it's the other bank. Very sporadic. I may just go the anti-fouler route but it's just a nuisance right now.

Thanks @MikeiNH67. I asked the mechanic who did the upstream, since I didn’t have time, to look at the rest of the system and he said he didn’t see or hear a noticeable leak. However, I know there probably is one, because when I give it gas in neutral I can hear that classic deep note gurgling that sounds like a leak.

Is it possible though a clogged cat could also make that sound but no leak? I also have the manifold ticking like everyone but my understanding was that typically doesn’t throw a code.

One more piece of info: the light was coming on more frequent a few weeks ago, but not every trip out. I tried cataclean and after that the issue now seems worse.. could the cataclean fouled my downstream sensor? I replaced that one 10-15k ago. Prob not right, because I would be getting other codes? Man this is confusing.

Thanks for the help

your cat is going bad, there's no way around it. Clearing the code doesn't make it go away. Your system needs time to initialize and for the O2/Cat related issues the parameters you need to hit for system initialization take a bit of time (it needs to read various engine speeds/loads to confirm its good/bad) and just driving around town most of the time won't satisfy the requirement, hence why you see it when getting out on the highway.

The upstream sensor (at least in my experience) is rarely to blame with these. The downstream sensors do go bad, but if you replaced it and the issue is still there it just about confirms the cat issue (not 100% but very high).

The spacer on the downstream sensor sensor works. The issue is that it's not a one-size-fits-all solution. You can't just throw a spacer on and call it good. It depends on how bad your cat is (ie how much/little O2 the sensor is getting) which will impact the reading.

What I have found is pull the spacer out too far will get rid of the P0420 code but will then start throwing the code for the lower sensor resistance circuit. This is caused by the sensor not getting enough gas flow at lower speeds.

What I had to do was play with how far the spacer spaces out the o2 sensor while monitoring the resistance reading on the sensors via a cheap amazon obdii reader. I found that the only way to get mine to work (which it has been for about a year now since my last fix) is that the downstream sensor needs to show changing values at lower RPMS/partial throttle. If you're not seeing a pretty constant changing value on the sensor at lower throttle inputs you will likely throw another code. It's a bit of a pain in the ass, but once dialed in you should be good to go for a bit longer.

Thanks @aemravan
Really good info. Interesting on the “dial in” part.

So I did the spark plug Fowler fix today. Had to pay a machine shop $20x2 holes so I could get the o2 sensor to fit and open up the bottom hole for a little more flow. $40 bucks took him 5 mins lol. Oh well. All good.

So the light stayed off all day. Good sign but not out of the woods - as you mentioned.

Interesting observation though:

So the truck seems like there is ever so slightly less torque at the bottom range, but the power band feels a little smoother. I definitely have to press on the accelerator a bit more to get the torque.

At first I thought it was the computer resetting since I disconnected the battery, but it’s stayed that way all day. It’s slight but definitely quieter and little less low end torque, maybe like 5-10 ft lbs. total guess.

Any comments on why this would be. Also, is it bad to have one bank with this workaround and not the other bank of cylinders? This may seem dumb, but is there a downside to this fix like I could run too lean or rich and damage the engine?

I need this girl to last!! I do plan on doing the manifolds this summer just need to find someone that won’t crush me $$$

Do you have a obdii scanner? That would be a really easy, guess-free way to figure out if your light is going to stay off or not. Like I mentioned, the O2 sensor and Catalytic converter system initializations tend to take quite a bit of driving to get situated. With the scanner in you'd see right away if all systems are a GO or still initializing.

Depending on how long you drove for, and under which conditions, it may be leaning towards the positive side with the light not on.. if the sensor was really far out it tends to throw a code pretty quick.

Thanks @aemravan

Any idea why my throttle responses seems less sensitive. Seems like a little less torque in the 1.5-2.25 rpm range. Could spacing a downstream o2 cause this?

Did you notice less power? Still no codes fingers crossed. I just have a tool to check and clear, not scan I believe.

that's hard to say.. I can't say I noticed anything super obvious from before/after the fix..I also don't think (but far from 100% positive on this) that the downstream sensor aids in any adjustment of fuel trims, I thought it was there strictly for monitoring the efficiency of the CAT. The upstream sensor IS actually altering your A/F ratio based upon the readings its getting.

This is probably a silly question.. but you did space the very down stream sensor, and not the upstream, correct?

It could possibly be that now that it's not throwing codes that the closed loop calibration is readjusting itself.. also not confirmed, but I believe when you throw a code for the downstream sensor or the cat there is a slight adjustment to your fuel trims (possibly to richen the mixture to keep a lean condition from happening, that's way safer than running lean and hot). Maybe that's what you're feeling.. but obviously just an off-the-wall thought.

@aemravan

That was actually my thought as well. And yes definitely did the downstream.

But to your point my thought was perhaps I was running rush before and getting more low end power and worst fuel economy. Now that I’m not throwing that code perhaps the system recalibrated and it’s loss some low ended power. It has seemed like top mod range is better.

But it’s annoying because it’s switching gears more often to get to the power and. Most noticeable on steeper hills.

My other thought was maybe it’s running rich, because apparently at low end rpm it’s better to start slightly lean to maximize the AFR then add richness as rpm increases.

Either way it’s a little annoying. The first .5-.75 inches of the gas pedal is totally dead. Which seems weird to me and is definitely different than before.

Any other insights please throw me way. Thanks for all of the help.

If you are getting P0420 and have an exhaust leak, you can be pretty confident that it's from extra air entering the exhaust system and triggering the code. I have a cracked v8 header that triggers a P0420, spaced the appropriate downstream o2 sensor and no more code.

The fact that you have power loss issues means you might actually have a plugged cat. Here's something to try. Take an IR temp gun and temp the various cats and see if any of them are far hotter than they should be. Also, look at your short term and long term fuel trims if you have OBD2 capability.

I think you're good here but its worth saying: NEVER space the upstream sensors. That's how the engine determines fueling in closed loop (part throttle and idle essentially). Only the downstream sensors should be spaced to defeat the P0420 in the case of an exhaust leak triggering the sensor.

So, i am having a similar problem as well, mine throws the codes on both banks, usually on the highway, after about 40 to 70 miles. now i also feel the truck is a lil less responsive. how do i diagnose blocked CATS?...the temp gun u described, how would it tell me my cats are blocked, or that i have leaks on both banks?

I bought new manifolds with cats from Rock auto and installed the back that kept giving my code after replacing all maintenance items first.

Turns out it was likely the exhaust donut, but I also had the sulfur smell so could have been both. The donut was trashed though and I could hear the leak with windows down and next to a wall.

$400ish for the pricier cats but I'll hold onto the stock just in case

Earthrealm, both banks is kind of odd, are you sure it the inefficiency codes? If you have good pressure out the tail pipe then probably not clogged, I don't see that as something that happens often. You can put a rag in the tail pipe and listen for leaks under the passenger feet area. I found an exhaust manifold leak by putting a few pounds of air pressure into the tail pipe sealing it with a paint can cover and hose clamp then listened for the hiss.

BTW, I've noticed on Toyotas that higher octane fuel can make these codes go away for a bit, like long enough to pass emissions.