CEL Code P0420 Cat Convertor Low Threshold

[TheBude]

2002 Park Avenue. currently at 140k miles. Purchased three years ago at around 122k miles on it. The P0420 started about a month ago. It's likely below 95% efficiency but not fully clogged. Definitely looks like the original cat, exhaust pipe and maybe even the muffler as well from the exterior. It's has a lot of surface rust but without any noticeable holes or damage in the exhaust system. The PCV valve is working properly and no other codes or issues.

I found a decently priced Walker brand (new) replacement for the cat only. It bolts to the header exhaust upstream. However it's looks to be welder from the downstream side of the cat onward. I haven't brought it to an exhaust shop yet. My question is can the existing exhaust pipe be cut and coupled to the secondary (posterior) side of a new cat convertor?

My guess is the upstream side bolts will need to be cut-off and replaced as the are likely too rusted and possibly melded together. As a side note I did try a can of CataClean in about 1/8th (4 gallons) tank, but it didn't seem to help and the p0420 returned in a few days.

 

[ulycyc]

If you have no exhaust leaks I would first check the up and down stream O2 sensors with a scan tool. If original with your mileage I would replace both regardless. buy only AC Delco or Denso brand.

 

[TheBude]

If you have no exhaust leaks I would first check the up and down stream O2 sensors with a scan tool. If original with your mileage I would replace both regardless. buy only AC Delco or Denso brand.

Thanks for the reply. Though I don’t have records of it my guess is that that the primary (upstream) o2 sensor has likely been replaced. The secondary may be original.

If they can be removed without damaging the threads I would likely reuse at least for a while to see if they are providing good live readings with my OBD2 scan tool.

 

[HotZ28]

How much oil are you consuming in 3K miles? With that kind of car mileage, I would have to be concerned about leaking valve oil seals causing the cat & O2 sensor contamination. Waiting on a response from you.

 

[TheBude]

How much oil are you consuming in 3K miles? With that kind of car mileage, I would have to be concerned about leaking valve oil seals causing the cat & O2 sensor contamination. Waiting on a response from you.

That's a good question.
Over 3k miles probably a quart of oil. No noticeable oil leaks on driveway in three years of ownership. No smell of oil burning oil, such as leaks on to the exhaust either. No blueish-gray smoke or oil smell from exhaust noticed on startups. It does idle rough intermittently on morning cold starts until operating temp is achieved. Which for the age of the engine and likely small compression losses developed over time/use doesn't seem out of the ordinary.

I Googled leaking valve seals and this is the AI description "Leaking valve stem seals can cause various issues, most notably bluish-gray smoke from the exhaust, especially noticeable after the engine has been idling or sitting for a while. Other symptoms include increased oil consumption, reduced engine performance, and potentially fouled spark plugs." I had a P0300 recently. Testing via rpm drop test/HEI spark tester. I also ruled out the fuel injectors by pulling each harness off one at a time a listened/looked for rpm drop. Both test lead to one of the spark plugs as the cause. Replaced all six with Iridium's and no P0300 since. None of the old park plugs were visibly fouled with oil, gas or carbon buildup.

I watched this video on GM 3800 v6 valve stem seals replacement

Starting at the the 35:34 time mark he states that the engine has been burning a quart of oil every oil change (3k miles conventional oil). That aligns with my engine's oil consumption. Could very well be leaking valve stem seals causing contamination. Good call, sir! Very good video also.

 

[02LTD2.0]

If the Cat is not clogged you could use an O2 sensor extender to move the tip of the downstream sensor out of the direct path of the exhaust. Google Big Daddies garage.

 

[TheBude]

If the Cat is not clogged you could use an O2 sensor extender to move the tip of the downstream sensor out of the direct path of the exhaust. Google Big Daddies garage.

It’s called a double-defouler. Cat already purchased.

 

[02LTD2.0]

It’s called a double-defouler. Cat already purchased.

That's the really cheap way to do it using cheap spark plug anti foulers.
I passed emissions for 5 years with a "test pipe" in place of the cat using an extender. Hey.....it's your money. I just know what worked for me and it was only 25 bucks.

 

[HotZ28]

You can either fix the problem, of go for a cheap substitute band-aid alternative bypass! With the bypass, it will not get better & continue to clog the cat & foul plugs.

 

[TheBude]

That's the really cheap way to do it using cheap spark plug anti foulers.
I passed emissions for 5 years with a "test pipe" in place of the cat using an extender. Hey.....it's your money. I just know what worked for me and it was only 25 bucks.

My second car is a 1995 Honda Civic EX coupe. I know the options.

 

[TheBude]

Just an update. Took it to shop that replaced the Walker brand catalytic converter that I purchased. Part and labor was $275 total. The old cat was definitely clogged (checked with a flashlight and compressed air). The engine is running smoother and no stalling which was also helped by replacing the fuel filter.

Thanks for the replies.