2 weeks to figure out the P0420 CEL before smog + a 50 mile offroad shakedown

[Endlessblockades]

I was going to complete my bi-annual CA smog last month (right before I put on the MF Overland cat-back kit - "just in case", I thought). Well, I didn't get around to it and my smog is due this month, so I've got a couple weeks to figure it out. I read a bunch of old threads, but just wanted to see if anyone thinks that starting with the sensors is still the way to go. When I was bolting up the new MF pipe to the flange, I could see the 2nd sensor in there and said a silent prayer. Not saying there is any correlation whatsoever, it's just bad timing, but I am quite capable of making things harder for myself. CEL came on in the evening after a 600 mile freeway drive. Cleared it with my Ultra Gauge and it came back in a few hours the next morning. I did a shakedown 4X4 run on an easy trail down here in SoCal (Berdoo Cyn up to Joshua tree) and it never came on during the drive after another CEL clearing. It may be rated 'easy' but it was 50+ miles RT and 5000 feet of climb on dirt/river wash with a couple rock gardens and I did all the extra credit stuff along the way. Shook the F out of the rig, got some pinstripes and some grind marks on the 4XI skids. This was the first real off road run I've done of any distance since completing the new-owner maintenance stuff and making some suspension and armor mods (sliders and skids mainly). CEL came on back in civilization. Other than some wild transmission temp fluctuations I'll mention in another thread at some point (need an aux cooler - 245 degrees can't be good, right?), she handled business like a CHAMP and the engine temp never went north of 192 even on the most taxing climbs. Anyway, does anyone want to share recent experience resolving this CA smog-check killer? It would be easiest for me to start with the downstream sensor of course, but is it worth it to have a pro-level scan done to truly diagnose the issue? I do have trust issues with most mechanics......

 

[19963.4lsr5]

Are you having to add coolant to the expansion tank?

If your LT fuel trim is between -10 and 10 I don’t see what a scan will pick up.

Air leaks in the exhaust will also cause it besides antifreeze ingestion.

How old is your rear O2? Is it’s gasket dead?


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[Endlessblockades]

Are you having to add coolant to the expansion tank?

If your LT fuel trim is between -10 and 10 I don’t see what a scan will pick up.

Air leaks in the exhaust will also cause it besides antifreeze ingestion.

How old is your rear O2? Is it’s gasket dead?


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1. No, the level remains constant. System has also been burped to bubble-free a week ago, before my trip.

2. I will need to pull those numbers.

3. Air leaks prior to the sensor would be a problem, but not after the 2nd cat, right? I did bolt on the MF cat back kit but I should recheck the clamps anyway...

4. I've only owned the rig a year and a half or so and I've never really inspected the sensor....

 

[19963.4lsr5]

Air leaks after the second Cat can still be a problem. There is reversion that will cause air to be drawn in.


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[Endlessblockades]

Air leaks after the second Cat can still be a problem. There is reversion that will cause air to be drawn in.


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Thank you. That's where I'll start. I'm still on vacation with the family but will find a corner of the local walmart parking lot and check all the clamps....

 

[3bears]

following :
I too have the 420 code and it comes back within minutes of clearing. Have new denso sensor, just no place to do the work right now and am supposed to get another 15 inches of snow over the next few days.
when I bought it it had the code and got it as a cheap fixer upper, it had a bad coil and a plug not firing at all, a filthy air filter. so it got new plugs, wires, 1 coil, filters, oil change and cleaned the maf and throttle body, pcv valve and reconnected a vacum line that was off. yet it still throws the code within minutes. so I wonder how much unburnt fuel went into the cat and what damage that could have done. Hope its just the downstream sensor.
does not burn oil ( but leaks out the vc gaskets) compression and leak down were good, does not use antifreeze now that I fixed that issue, ( bad hose and cap). Other than the code it runs great, starts right up, idles normal, trans shifts like it should.. MPG is not a good judge, as it has to sit and idle a lot to warm up even wiht block heater, has snow tires and ive been in 4wd since I bought it 3 months ago....but im getting about 12-13. due to road conditions have not had it over 60mph, but smooth even at that point. 265k on the clock.

 

[Dezert4Runner]

Been dealing with the same thing for about 6-7 weeks now. My smog was due a month ago. My last smog 2 years ago I had no problems with the p0420 or my readiness monitors to complete, but have been getting a random occasional p0420 (mostly during/ after offroad trips or long road trips) for the last 8 months. I would say minimum 400 miles between codes being thrown.

I tried riding out the drive cycle a number of times hoping the catalyst would become ready and never did before the code would eventually come back. Just installed a Denso rear o2 sensor 3 days ago, and catalyst and evap readiness monitors are still not set because the p0420 code was last cleared about a week ago. I’m hoping that it ends up completing monitors but not optimistic about it to be honest. Just for back story, I replaced my front a/f sensor 50k miles ago also with a Denso unit, so I’m leaving that one alone unless someone can convince me to replace it again.

I’ve been meaning to pick up a shop vac with reverse function to use with soapy water in a spray bottle to test for bubbles because I have a sneaking suspicion it could be an exhaust leak. I’ll probably give the drive cycle an honest chance this next week to work before checking for vacuum leaks. After that it’s new cats! Yesterday I was quoted $525 + $425 for front & rear Magnaflow cats (mine is the CA emissions, not fedetal) plus $150 total to install both of them so I’m really hoping I can get it figured out before it comes to that. Saving up for long travel and this is the last thing I want to spend that money on right now.

2000 3.4 manual 4WD with around 400k

 

[gamefreakgc]

P0420 is pretty straight forward. It's normally a bad rear O2 sensor but can also be an exhaust leak (though these tend to leak out exhaust gases, not suck in fresh air), or a bad cat.

If you have to replace the cat, look at Magnaflow's CA smog legal offering. I have one on my 4Runner and it passed smog with near 0 emissions.

 

[photoleif]

definitely change out that rear sensor. in an attempt to combat the P0420 i changed my front one to no avail (and shouldn't have even started there, i've since learned). i've also been far more regular with cleaning the MAF sensor, but again i've learned that is probably not really the cause. the P0420 diagnostic pages in the FSM have about four causes to troubleshoot: 1) exhaust leak; 2) front sensor; 3) rear sensor; 4) cat.

 

[HiLife]

The o2 sensor wear over time and become slow responding, needing replacing as a maintenance item.
Would be good if you could read what the sensors are doing with a o2 reader if you have one, before replacing them to make sure they are out of spec.
I was troubleshooting a 420 on another Toyota, and the owner wanted the sensors replaced thinking it was the cause of his 420. My Ultra gauge said the sensors were working and my temp gun measurement before and after the cat pointed to a dead cat, and told him the sensors wouldn't work for him.
Replaced them anyway at his direction, and it didn't make his 420 go away.
He was saving for a cat when he totaled the car so I never found out if replacing the cat would've worked.

 

[photoleif]

The o2 sensor wear over time and become slow responding, needing replacing as a maintenance item.
Would be good if you could read what the sensors are doing with a o2 reader if you have one, before replacing them to make sure they are out of spec.
I was troubleshooting a 420 on another Toyota, and the owner wanted the sensors replaced thinking it was the cause of his 420. My Ultra gauge said the sensors were working and my temp gun measurement before and after the cat pointed to a dead cat, and told him the sensors wouldn't work for him.
Replaced them anyway at his direction, and it didn't make his 420 go away.
He was saving for a cat when he totaled the car so I never found out if replacing the cat would've worked.

that's definitely the right way to investigate this. i've been throwing small money at it in hopes of a resolution that doesn't involve a $1500 cat, but i realize that may simply be getting me timely replacement of wear items.

 

[Endlessblockades]

Thanks for all the excellent advice everyone. I may have gotten lucky for the time being. I found a couple loose exhaust clamps and looked up the torque specs (about 40 ft lbs from what I saw during a brief search) and had a torque wrench with me and redid all the bolts. None were that tight. I had been swapping things around to make it quieter for the fam. I'm feeling optimistic because the first time I got the code the other day and cleared it, it returned within 20 miles. I just drove 500+ miles from SoCal with multiple stops and conditions and I have no Pending code. I'm going to look at any emissions data I can find with the UG and if it's within spec get tested ASAP. Hopefully the amount of miles I just put on will satisfy the data collector and not appear as being recently cleared to the smog machine. If it returns before or after I get smogged, I will begin with the rear O2 sensor. Rear cat is very hot to the touch so It appears to be doing its job for now....

 

[Skulking]

Thanks for all the excellent advice everyone. I may have gotten lucky for the time being. I found a couple loose exhaust clamps and looked up the torque specs (about 40 ft lbs from what I saw during a brief search) and had a torque wrench with me and redid all the bolts. None were that tight. I had been swapping things around to make it quieter for the fam. I'm feeling optimistic because the first time I got the code the other day and cleared it, it returned within 20 miles. I just drove 500+ miles from SoCal with multiple stops and conditions and I have no Pending code. I'm going to look at any emissions data I can find with the UG and if it's within spec get tested ASAP. Hopefully the amount of miles I just put on will satisfy the data collector and not appear as being recently cleared to the smog machine. If it returns before or after I get smogged, I will begin with the rear O2 sensor. Rear cat is very hot to the touch so It appears to be doing its job for now....

I don't know if the Ultra Gauge can do it, but if you have a decent OBDII scanner you should be able to check smog readiness on it so you know ahead of time whether they will test your car or not.

 

[FD7683]

If it's the cat, then maybe throw a bottle of CataClean to see if it does something? I've heard nothing but good things about it.

 

[Endlessblockades]

2 months later & after a very successful Star smog check, the P0420 CEL has not returned.

Exhaust leak BEYOND the downstream O2 sensor was the cause. Leak was due to my hasty assembly/reassembly of the MF Overland cat-back exhaust system.

 

[4runnerfiend]

Pesky P0420. mine will do the same thing come on after a long road trip/offroad trip. and then it will go away after clearing. I've determined it to be an exhaust leak coming from my down pipe. bolts like to loosen up there for some reason. Hopefully timing will be in my favor when smog comes up again. Funny to hear this is a common thing on the 3gt4r

 

[Dezert4Runner]

I also resolved mine after finding exhaust leaks. Soapy water and shop vac worked like a charm!

I had a visible gap on the flange between the bank 1 header and the Y pipe, but just on 1 section of the 3 sided flange. The bolts were as tight as they could be, it was just warped. High temp Permatex RTV did the trick (the 2000 degree rated muffler & exhaust one, not 600-700 degree “high temp” one). IDC if I have to do it again in 2 years, as the only way to permanently fix (I believe) is to remove the header to get to the 3rd bolt on that flange since it’s tucked between the engine and the y pipe. F that. What a bad spot for accessibility.

There was a second leak at the upstream A/F sensor that was replaced 4 years ago with a Denso unit. It was just a bad gasket. I grabbed a Napa replacement to resolve that one. Cheap and easy.

Exhaust leaks even on non rust buckets are a common issue as these vehicles age, it appears.

 

[HiLife]

I also resolved mine after finding exhaust leaks. Soapy water and shop vac worked like a charm!

That's a good way of finding exhaust leaks!! That could come in handy when time comes to troubleshoot an exhaust leaks.