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Old 08-02-2021, 03:50 PM   #1
paulsome
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2003 ION-3 Sedan
Default P0420 code after new sensors/cat

Hi all.

I've got a check engine light code that won't seem to go away. The code is P0420 (Catalyst System Efficiency Below Threshold Bank 1).

Some quick background - the car is an '03 Ion 2.2L with the 5spd manual, currently has about 255,000 miles on the clock. About two years ago, the exhaust completely rusted out just downstream of the manifold/catalytic converter flange. So I replaced the entire exhaust - catalytic converter included - from the manifold back.

Around this time, I was getting codes P0420 and P0137 intermittently. I replaced the downstream O2 sensor a few months later, and if memory serves, only the P0420 code came back. I figured it was because the upstream O2 sensor was now way older (I had never replaced it in the 8 years I had owned the car at that point).

I finally just recently replaced the upstream 02 sensor, and over the next thousand miles or so, the P0420 code cleared itself, came back, cleared itself again, then came back on steady. I manually cleared the code, and am waiting to see if it returns (which it probably will).

My question is, why would this code keep coming back? Everything I find on the potential causes for this code direct me to likely issues with the upstream sensor, and if not that, potentially the cat or downstream sensor. All three of these have been replaced relatively recently now. Am I missing something? It seems unlikely the cat went bad after only 2 years, but maybe the downstream sensor crapped out?

Thanks for any assistance you can provide.
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Old 08-02-2021, 07:23 PM   #2
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Default Re: P0420 code after new sensors/cat

Original catcon or aftermarket?
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Old 08-02-2021, 08:01 PM   #3
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Default Re: P0420 code after new sensors/cat

I replaced the catcon with the rest of the exhaust, since where it rusted out was part of the catcon itself (the pipe portion between the manifold flange and the catcon itself). It actually fully separated while driving one day, so it was essentially dumping raw exhaust directly from the manifold.

I'm fairly certain I ordered all the replacement parts from Rock Auto, and I usually spring for at least the mid-tier parts - my experience is the cheapest option is usually cheap for a reason.

So that's why I'd be surprised that this replacement cat which is only 2 years old went bad. I haven't even driven the car that many miles since I did the work. I've gone through a number of downstream O2 sensors over the decade I've had this car, so my thought was maybe that went bad even though it too is only 2 years old?
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Old 08-02-2021, 08:41 PM   #4
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Default Re: P0420 code after new sensors/cat

Side story. Relatively early in my L300s life, around 75k miles, each upstream O2 sensor decided to protest. L300s are 3.0L V6 engines requiring four O2 sensors due to two exhaust manifolds. Both error codes occurred about a year apart for the heater, not the main operating part. The first one occurred in winter and I (sheepishly) wanted to believe contamination from snow (they're high up on the exhaust manifolds). Periodic resetting was temporary until warm weather returned to look into this. Spring rolled around and I was forced to replace the first one with a oem replacement ($$). Almost a year later the second upstream O2 sensor triggered the same error code for a faulty heater. I didn't hesitate replacing it. Both replacements have about 60k miles on them. No O2 error codes since replacement.

P0137; https://www.obd-codes.com/p0137
P0420; https://www.obd-codes.com/p0420

Each error code definition runs deep into emissions, operation, wiring, overall engine health and catcon function. I was lucky(?) the upstream sensors were the issue. Your dilemma involves the catcon and how efficient it is whether original or replacement. Offhand I do not know how to test catcons for efficiency when an error code points to diminished catcon operation. The downstream sensor measures catcon efficiency. Maybe the older exhaust gas analyzer (sniffer) can help determine if a catcon is working or worn out? This presumes wiring and ecm are operating correctly.

It can become complicated troubleshooting post catcon O2 sensors, wiring, and catcon.
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Old 09-02-2021, 12:20 PM   #5
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Default Re: P0420 code after new sensors/cat

UPDATE - Still have the code, and when I manually reset it, the code comes back pretty consistently after the catalyst monitor runs its drive cycle check.

Will this P0420 code be thrown if the car is California emissions certified from the factory, and a federal/EPA-certified (i.e., non California-certified) aftermarket catalytic converter is installed? I heard recently that the car can tell the difference and will throw the code if it detects the catcon is not a California-certified model. Has anyone heard of this before or know if there is any merit to it?

The catcon I installed about two years ago was the cheaper EPA-certified model. Now that I seem to have exhausted all other reasons for this code showing up (see below), it seems like a logical explanation to what is happening here.

Car isn't misfiring. No leaks/holes in the exhaust. Catcon appears to be functioning properly. New 02 sensors and coolant temp sensor. Car runs/drives fine, doesn't appear to be an issue with fuel pressure or oil contamination in the exhaust.
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Old 09-02-2021, 09:41 PM   #6
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Default Re: P0420 code after new sensors/cat

On rockauto's site, there's a page describing CARB and EPA compliance related to catcons; https://www.rockauto.com/genImages/c...html#Emissions. Check your emissions label for specific info on CARB, NYS or EPA compliance. This will give you a clue which way to go. Whether or not CARB emissions control implies a different, more stringent emissions control program for CARB, NYS and other states following California higher emissions bar from federal EPA emissions may be answered by GM.

If you compare CARB to EPA catcon reduction values, the only difference seems to be a 30% higher NOx reduction for CARB catcons over EPA catcons.
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Old 09-05-2021, 05:43 PM   #7
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Default Re: P0420 code after new sensors/cat

Carb cars need carb parts
the pcm is programed differently on CARB/PZEV Models it expects a different sensor voltage feedback then a non carb car afaik the carb cats run a bit hotter and behave differently during warmup

you could try the old non-fowler trick and see if it is enough to get the sensor to make the pcm happy
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Old 09-09-2021, 07:51 PM   #8
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Default Re: P0420 code after new sensors/cat

Quote:
Originally Posted by rjc862003 View Post
Carb cars need carb parts
the pcm is programed differently on CARB/PZEV Models it expects a different sensor voltage feedback then a non carb car afaik the carb cats run a bit hotter and behave differently during warmup

you could try the old non-fowler trick and see if it is enough to get the sensor to make the pcm happy
Thanks for the info. The few times the check engine light cleared on its own was during long haul highway stints of several hours or more at a time -figured maybe at those points the cat was just getting hot enough to run more efficiently and clear the code.

I considered the non-fowler angle, but another caveat: I used a temp gun to read the upstream/downstream temperatures and noticed I was getting hotter readings upstream (~ 525 degrees) than downstream (~ 485 degrees), which indicates poor performance. Coupled with the fact that when I manually clear the P0420 code, and it regularly comes back after the catalyst drive cycle monitor runs its check, it seems like the cat actually is under performing.

Since I feel like I can rule out any underlying issues that would have caused the cat to fail prematurely, I'm leaning toward maybe I just ended up with a defective cat. Since it appears I need a new one anyway, I guess I'll bite the bullet and try to find a CARB certified cat that doesn't cost more than the car is worth. And maybe avoid buying another Walker brand cat this time lol.
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Old 09-09-2021, 08:36 PM   #9
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Default Re: P0420 code after new sensors/cat

Using an infrared heat gun to measure before and after catcon temps is another quiver in diy diagnostics. Any chance of getting a previously owned oem NYS or Calif catcon from an Ion in nearby junk yards?
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Old 09-10-2021, 09:25 AM   #10
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Default Re: P0420 code after new sensors/cat

Quote:
Originally Posted by fdryer View Post
Using an infrared heat gun to measure before and after catcon temps is another quiver in diy diagnostics. Any chance of getting a previously owned oem NYS or Calif catcon from an Ion in nearby junk yards?
I may check out a few local junkyards to see what's available. I miss having a U-pull it yard nearby - the parts were usually way cheaper. If nothing else, I found an Eastern Exhaust CARB certified cat online for around $550, which isn't terrible.

Just for laughs, I looked at the GM parts direct website to see what the cost of an OEM cat would set me back. Turns out they're discontinued and no longer available for purchase, at least for 2003/2004 models.
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