What causes catalytic converters to "go bad"?

Cracker · Postby **Cracker** » Tue Jul 22, 2014 8:02 pm

I have a 2005 PF. I'll hit 140,000 miles sometime this week. The check engine light has been on for a couple of months and I can smell the excess fuel when I back into the garage and park. The diagnosis is that the two front cats need to be replaced. In CA that's anywhere from $2100 to $2500 parts and labor. One garage asked me if I "knew what was causing the cats to fail"? Is there something specific or do cats simply wear out over time?

Though my gas mileage is, and has been in the 13-14 normal driving (average 30 mph) and 17-19 on the highway, I haven't noticed a change in power, acceleration, or idle speed.

Postby **smj999smj** » Tue Jul 22, 2014 10:33 pm

Catalytic converters can fail in several ways. First, they can rust out and leak. The catalyst substrate can become inefficient at breaking down the exhaust emissions, which will tend to cause the P0420/P0430 codes to set. They can become clogged or melt internally, which is usually the result of an engine that is burning oil or coolant, or running too rich. An engine that is running too lean is also not good for them.
It's not uncommon for catalytic converters to become inefficient over time and it's a fairly common incident on the R51 Pathfinder. If you are smelling excessive fuel, you may want to investigate further before investing in one or more catalytic converters. Make sure there are no fuel leaks and test the fuel pressure with a fuel pressure gauge to make sure it's within specs and not excessive. Worn spark plugs could also be an issue. It would help to know what the trouble codes were.

Cracker · Postby **Cracker** » Wed Jul 23, 2014 3:33 pm

Thanks for the prompt answer. The codes are P0420 and P0430.

The mechanic in this video suggested putting a gallon of lacquer thinner into a half take of gas and driving for 30 minutes at highway speeds.
http://www.youtube.com/watch?v=5icTmYItwiE
He claims this can clean much of the jumk out of the cat. Would there be any harm in doing so?

CPLTECH · Postby **CPLTECH** » Wed Jul 23, 2014 6:22 pm

I have to agree with smj999 about the plugs. Even tho they are supposedly good for 100k, I replaced mine before the 90k mark because of some comments on the forum.

Postby **disallow** » Wed Jul 23, 2014 8:11 pm

I am experiencing the P0420 and P0430 codes on my 05. I have 150000mi on it. I think its either:, and have been monito

1. Bad secondary O2 sensors.
2. Bad Cats.

I've experienced no change in fuel economy or performance and have been monitoring the voltage on my rear O2 sensors. The voltage seems to be all over the map, varying from .1 to .9 volts, moving randomly up and down.

I've never changed the rear O2 sensors, only the primaries.

However, if my cats end up being the problem, I am contemplating having them hollowed out. Would make sense to also remove the secondary cats at that point as well? I worry about overloading them...

Cracker · Postby **Cracker** » Thu Jul 24, 2014 10:21 am

I do like the info I get off this forum. I trust the regulars on here more than I ever will a mechanic. But sometimes it's too much info, you know what I mean!

I golf and ride street and dirt bikes on the weekends. I don't mess with my PF in my free time. In CA I think the O2 sensors are inside of the cat (emission regulations), which makes them so expensive. It's easiest for me just to have the two cats replaced and the spark plugs and call it a day. Thanks and have a good weekend!

--David

Postby **smj999smj** » Fri Jul 25, 2014 5:46 am

Rear O2 should be constantly cycling from near 0 volts to approximately 1 volt, just as the front sensors due. The difference between a bad cat and a good cat is the frequency of the cycling of the rear sensors should be approximately half that of the front's if the converter is good. Gutting out the converters will not correct a P0420/430 code. As far as the rear sensors being bad, they will usually trigger a rear O2 sensor trouble code if they are bad.

I don't recommend pouring a gallon of lacquer thinner into the gas tank. If the catalytic converters are inefficient, the only way to fix them is to replace them. Rear catalytic converters are usually not monitored and rarely fail as the front converter sorta acts like a filter in relation to the rear cat. Most contaminants that might harm a converter will get caught up in the front converter.

Postby **disallow** » Fri Jul 25, 2014 5:51 am

Yeah, I realize hollowing out the converters would cause a constant p0420/430 code unless I got some O2 sensor simulators.

I'm more concerned that if the cats are bad, they'll break down and take out my engine.

Also of note, my front o2 sensors don't change voltage very much. Bank 1 is at .3v and bank 2 is at .6 without much variation. I'll try to take another video of the readout and share.

Postby **disallow** » Fri Jul 25, 2014 5:58 am

Cracker wrote:I do like the info I get off this forum. I trust the regulars on here more than I ever will a mechanic. But sometimes it's too much info, you know what I mean! I golf and ride street and dirt bikes on the weekends. I don't mess with my PF in my free time. In CA I think the O2 sensors are inside of the cat (emission regulations), which makes them so expensive. It's easiest for me just to have the two cats replaced and the spark plugs and call it a day. Thanks and have a good weekend!

--David

Agreed. If I lived in California I wouldn't even think about gutting my cats. I don't recommend to anyone really, as I'm sure doing so could net a significant fine if caught, regardless of where you live. Higher chance if getting caught where they do regular emissions checks, though.

HydroPonix · Postby **HydroPonix** » Mon Jul 28, 2014 9:37 pm

I took my pathy to a small muffler shop and told the guy about my codes and how it sounded like tincans rattling under the car not to mention the deeper resonance when i accelerated. I was also popping codes for lean fuel to oxygen ratio and bank 2 o2 sensor. When he lifted it i went under the car with him and in 5 seconds he pointed out the the cat mount on the passenger side had separated. He welded it in less than 10 min and charged me 25 bucks. He said that extreem heat will melt/burn up the "paper" liner around the ceramic very quickly and thats when you start looking at the thousands to replace the cats. Id take it to a muffler specialized shop, ask what a diagnosis will cost (if they are slow they might just look at it for free) and ask what they are looking for in terms of damage and how much for a quick repair, it could save you a ton of hassle and cash. If it ends up being a serious cost you could even look for junkyard parts. I know its a risk and a bit time consuming bit in the end will save you some cash.

Postby **smj999smj** » Tue Jul 29, 2014 2:58 pm

It's illegal for salvage/junk yards to sell used catalytic converters, unless it had been properly tested and labeled that it has passed emissions testing. Most don't have the equipment to do so. It is also illegal for a shop to install a customer supplied, used converter if it has not been tested and labeled.

HydroPonix · Postby **HydroPonix** » Tue Jul 29, 2014 4:04 pm

Lol well said. Dont get jy parts for your cat system lol

dcudney · Postby **dcudney** » Thu Aug 21, 2014 9:25 am

I had to replace my 2005 pathfinder cats at around 160,000. As soon as I replaced them everything went back to normal.

What causes catalytic converters to "go bad"?

What causes catalytic converters to "go bad"?

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