P0420 and P0430

Back again folks. I’m getting the dreaded P0420 and P0430 codes. Been doing a lot of reading and it seems the consensus was that these two showing up together is usually a red herring for bad upstream sensors. To be thorough, I replaced all 4 sensors with Denso’s and yet I’m still getting the codes. I don’t have a fancy reader but what I have noticed is that the two upstreams read the nearly identical outputs under load and idle.

The downstream, though once all over the place, are both now near identical under load; however at file, B2S2 is still erratic. Not nearly as much before the replacing, but still so.

Open to any suggestions. Luckily my county doesn’t perform monitoring anymore so I have all the time to figure this out at the expense of maybe burning more fuel and possibly a cat. I’m leaning more toward there being a bad cat myself

 

Them showing up individually is likely a sign of one sensor on the impacted bank being being bad, and I'd replace both upstream and downstream on the affected bank.

Seeing them show up together is a whole other can of worms IMHO. I'd expect to see that if the truck had aftermarket cats, aftermarket headers/exhaust, a ground/wiring fault, or potentially other stuff in that vein (exhaust leak, or similar). We've seen people post that throw both codes and used aftermarket cats or are running shorties/long tube headers.

If it was a bad cat, I'd be shocked to see BOTH clogged at the same time, unless a previous owner (or original owner) ran with bad upstream sensors or some other condition that would cause excess clogging of the cat.

I'd start by really taking a close look at wiring based on your middle paragraph.

 

Man I really hope it isn’t wiring. That seems like a major pain in the ass to re-wire if it is. I will say B2S1 was surprisingly easy (ish) to get to. Don’t know if that’s factory design or not. It didn’t look tampered with though. I’d say it also be strange if both banks had bad wiring, no?

 

I'm thinking more along the lines of ground issue for wiring. It'd be something that was shared between both. But I think a leak is more reasonable. Do you know if the cats are OEM? Any evidence they've been replaced or had been cut out/stolen at any point?

 

Those codes can also be thrown by intake vacuum leaks, so check for those as well. Or even exhaust leaks can cause that too

 

No cats look stock. The triangular flanges also are the most rusty spot on the ole gal despite being a relatively clean frame truck.

 

I thought about this but I didn’t see any infiltration via the IAT, Throttle response and MAF live data.

I did have an Expedition in the past that had manifold leaks and ticked. I have noticed what sounded like a manifold leak but I can’t find a crack anywhere

 

Check hoses for cracks visually or spray around injectors and intake with carb or propane. Power steering and brake booster use vac as well

 

I mean, I feel like that's pretty typical. Anything iron from the heads to just past the cats is seeing the most heat and tends to rust/freeze on anything, in any climate. Is it possible you may be leaking at any point between the two sensors?

 

I thought that but I didn’t see anything. I’ll try to get another look tomorrow. Like you said, it’s weird that both bank’s sensors and their sisters are behaving near identical on the live data. That’s quite the coincidence.

 

I’ll get some carb clean and check it out. I doubt that’s the case but you can never be sure, especially with a vac leak

 

Be 100% sure that you do not have leaks anywhere in the exhaust system. The connections between each pipe must be tight and the gaskets at the fitting must be sealing. I've had codes on two different occassions that were caused by a loose connection between the y-pipe (after the cat) and the next pipe that connects to the muffler. I also had a code because one bolt holding an oxygen sensor had come loose.

 

(each causing air (oxygen) to be drawn into the system and triggering a code)

 

I mean, in theory, you would expect the upstream to be nearly identical, combustion should nearly identical between both banks. And in truth, what's seen on the other side of the cat should be identical also, if both cats are identical. But say someone replaced or even just removed the cats at some point, and there's a gasket required between the flanges, but it had disintegrated so the person doing the work didn't realize one was required ... or the O2 sensors are supposed to use a gasket. Or maybe on of the three studs on the flanges is broken, but you can't see it. Lots of stupid shit could happen to allow air in.

This isn't me telling you to go tear it all apart. Just spitballing some scenarios where I could see the ECU getting bad reads from the sensors.

 

Thanks fellas. I couldn’t find a compromise in the system anywhere. The flange bolts were a little loose when I PB pissed the surface crud off. Waiting to see if that will do anything. Like I said, we don’t test here so it’s not urgent but I’m OCD and seeing that light on with the possibility of getting even worse mileage definitely screws with my mind.

The only other off-hand scenario I can think of is that the original owner Pitt a cold intake on and moved the MAF location. The second owner (his son ironically) took it back to stock before I got it. Right back in stock’s location. However I figured if there was an issue there it’d be either with the sensor or the intake itself.
@FrenchToasty i checked and saw no intake issues. MAF is clean and live data reads smooth as well.

 

Just for shits and giggles, if you've never had the negative battery cable off for 10min, you may want to do that, just to force a re-learn. Not remotely suggesting it'll fix issues - may make codes go away. But will at least force a baseline relearn of EFI operation. You should do same if you kill the CAI.