Can you REALLY know if a catalytic converter is bad...

[Pandasaurus]

By temperature?

I was watching some videos on Youtube (learning how to replace a knock sensor, yay!), and happened across a video ( <Link! ) claiming that you can find out if your catalytic converter is bad by checking the temperature (using an IR thermometer) of the cat (close to the O2 sensor) and the pipe before (close to the engine) and after the cat. Video says pre-cat should be ~300F,a good cat should be ~500F.


If you've been reading my previous threads, you'll know why this interests me. And it's not only because I'm a total car noob. If you haven't: The reason this interests me is because I have a P0420 (cat efficiency below threshold, yay!), which from my reading says that could either mean the cat(s) is/are bad, or one or both O2 sensors could be bad. The vehicle ('02 Impreza 2.5TS, almost 208K miles) was recently in the shop for some unrelated repairs, and the shop said it's for sure the cats, but I'm not sure how they came to that conclusion, since we didn't ask them to do anything related, and didn't pay them for any diagnostics.

So, after a quick drive to the store and back (about 4 miles round trip, mostly highway), I grabbed the trusty Harbor Freight IR Thermometer, crawled under the car, and checked... Closest spot I can reach to the engine was ~350-400F, closest I could get to the O2 sensor was ~550F, post-cat was ~350-400F.

Anyone have any knowledge/experience with these sorts of things? I'd really rather my O2 sensors were bad than replace one or both cats, since I can get both sensors for about the same as one cat. lol

 

[Vdubchaos]

Of course the shop will tell you it's the Cat. It's the hardest/most work intensive work they can charge you for. Cats are NOT cheap.

No clue on the temperature thing, never really heard of it.

I would suggest that you do research on Subaru forums that's specific to your engine/model. I'm SURE there are people out there that had similar issue and will give you an idea of what you are looking at. Don't create a thread though, just find out what other's have posted (simple google search should work too).

I always like to start off troubleshooting from the easiest > hardest. Assuming some post success with simple O2 sensor swap, I would start there. In general O2 sensor is pretty easy to change assuming it's not in the PITA spot (which might apply to your car, I don't know). You can rent a special socket from Autozone for free...

I'm pretty sure you can easily check if it's the cat by simply unscrewing the O2 sensor and letting it hang (bypassing any Cat reading). Plug the open hole with a washer/bolt of the same size.

At that mileage I wouldn't probably bother with Cat swap. Assuming you have success with O2 sensor removal/plugging......leave it that way and clear the code.

Unless the research tells you otherwise, start with the post Cat sensor.

I had a similar issue when I put a test pipe on my passat. I was getting O2 sensor/cat codes.I removed the post cat O2 sensor all together and plugged the hole with a bolt. It did the trick for me.

 

[XavierMace]

I told you that thermometer was handy.

 

[phucheneh]

I would not trust a HF thermometer to measure those temperatures. It's hard to get an IR unit that will read high enough...500*F is actually pretty cool for a converter.

The test is to see if the outlet in definitively hotter than the inlet. That's not a myth; it's just hard to test in a way where I would actually trust the result.

As the name implies, the converter is using the substances inside itself as a catalyst for a chemical reaction. HC and CO to H2O and CO2, generally. That generates heat, which is why misfires are so bad for cats: a big pile of hydrocarbons (unburned fuel) + very hot converter = very very very hot converter (melted catalyst material).

Seems like I already gave some advice on this, pretty much agreeing with your shop's 'diagnosis,' a.k.a reading of DTC's. This is one of the few codes where it usually can just be inferred that the ECM is saying 'replace this part.' Nine times out of ten, it's the converter. Using my own line of thinking, and trying to give them some credit: they saw a car that ran fine, did not appear to have any kind of exhaust leak nor exhaust restriction. It had high miles. It had P0420 stored.

Put those things together; bad converter is overwhelmingly likely. Faulty O2's are not.

Remember what that code is saying: too much switching at rear O2. Bad O2's, more often than not, exhibit a slower than normal response time, which is the opposite. And if it's simply a voltage that is too low or too high (without the increased switching), it should just set an O2 code instead of P0420. Should. Don't 100% hold me to that...

 

[Ferzerp]

Kind of unrelated, but how common is running intentionally extremely rich (as in you can smell it in the exhaust) when high cat temps are detected in order to cool it down? I know my car does it, but do others?

 

[JCH13]

Kind of unrelated, but how common is running intentionally extremely rich (as in you can smell it in the exhaust) when high cat temps are detected in order to cool it down? I know my car does it, but do others?

Um, wut?

Excess fuel will be burned in the catalytic converter, causing it to heat up even more. I surely hope your car isn't actually doing that...

 

[Ferzerp]

Um, wut?

Excess fuel will be burned in the catalytic converter, causing it to heat up even more. I surely hope your car isn't actually doing that...

They most certainly do. I was just wondering how common it is.

http://www.diablosport.com/forum/viewtopic.php?f=26&t=36309

for an example explanation


I don't know that I've ever kicked it in at the track (which is really the only place you will), but others have had following cars note the occasional odor of fuel.

edit:

Apparently GM does it with at least the LS1 too:

http://forums.corvetteforum.com/c5-scan-and-tune/388592-catalytic-converter-overtemp-table.html

 

[twinrider1]

Is a pressure differential test more useful?

 

[thelostartof]

If you are getting that error code and there is a temp difference in the cat (you should measure at the front of the cat and then at the back of the cat) is less than 80-90+degF between the front and rear then good chances are the cat is bad. The rear of the cat should be 80+ deg F hotter than the front of the cat.

Also make sure you realize that there is a good chance that something else might of failed or is failing that caused the cat to fail. Could be failing 02, MAF going bad, or even worn out plugs not fully burning the fuel that could of caused it to run a little rich which caused it to kill the cat.

Make sure to measure after a good 15-20 min drive of driving the car a little harder than normal.

 

[SparkyJJO]

Not sure what temp is optimal, but I do know if your cat is glowing red that it is bad

Definitely want to check into if something else has caused it to go bad however. While they can just go bad usually they are helped along by something else wrong, such as too rich a fuel mixture.

 

[JCH13]

They most certainly do. I was just wondering how common it is.

http://www.diablosport.com/forum/viewtopic.php?f=26&t=36309

for an example explanation


I don't know that I've ever kicked it in at the track (which is really the only place you will), but others have had following cars note the occasional odor of fuel.

edit:

Apparently GM does it with at least the LS1 too:

http://forums.corvetteforum.com/c5-scan-and-tune/388592-catalytic-converter-overtemp-table.html

That flies directly in the face of:

http://www.freeasestudyguides.com/catalytic-converter-overheated.html

An overheated catalytic converter is usually the result of excess hydrocarbons (HC's) in the exhaust stream. [i.e. unburnt fuel]

http://www.doityourself.com/stry/3-common-catalytic-converter-problems#b

The catalytic converter is linked to one or more oxygen sensors in a modern vehicle. The oxygen sensor is responsible for measuring the fuel mix in terms of air and liquid fuel. When the mix is too rich, this can lead to overheating the cat.

http://www.bobsmuffler.com/convfail.html

An engine that is performing at peak efficiency will burn all the fuel in the combustion chamber during the combustion process. An engine that is not performing properly, that is not burning all the fuel, will allow unburned or excess fuel to enter the exhaust system. When this excess or unburned fuel contacts the hot core of the converter it will ignite. This constant infusion of unburned fuel will cause temperatures to continuously rise above the designed operating temperature until the core of the catalytic converter will actually melt. Possible causes for the excess fuel entering the exhaust system are an incorrect fuel mixture, incorrect timing, corroded spark plugs, worn and cracked ignition wires, improper fuel pressure, a faulty oxygen sensor, sticking float, faulty fuel injector or a malfunctioning check valve.

And there are many other sources like these.

 

[Ferzerp]

No, I understand it is odd, but when you have the people programing the tuners saying it does it (and is programmed to), many people observing AFR dropping, etc. It's hard to get a good picture of exactly what's going on. I was trying to find more info. Is it flooding it with more fuel that it could combust (making it oxygen starved) so the fuel acts as a coolant (it's really the only type of material that could be used as a coolant available in that system), or what?

 

[JCH13]

It might be a more subtle effect than what I was picturing. Having thought about it for a little bit... the cat over-temp correction might be bringing AFRs down only a tiny, tiny bit (by adding just a *little* more fuel) to reduce NOx and other lean-related emissions at the expense of fuel efficiency.

That is to say the correction is not making the car run pig-rich, but is sacrificing a little fuel economy for reduce the workload on the cat.

 

[Ferzerp]

I dunno, AFRs in the 10.5 range seems "pig rich" to me. Am I wrong?

 

[JCH13]

I dunno, AFRs in the 10.5 range seems "pig rich" to me. Am I wrong?

10.5 is pig rich... and I believe afrs like that can kill cats

Something seems off about all of this...

 

[JCH13]

OP if your car is OBD2 equipped you can likely check out your catalyst temperatures. I'd give that a try to diagnose your issues.

An OBD2 scan will also read your O2 sensors... if one is reading something insane like 29 or 7 AFR that will be illuminating.

 

[hans007]

Of course the shop will tell you it's the Cat. It's the hardest/most work intensive work they can charge you for. Cats are NOT cheap.

No clue on the temperature thing, never really heard of it.

I would suggest that you do research on Subaru forums that's specific to your engine/model. I'm SURE there are people out there that had similar issue and will give you an idea of what you are looking at. Don't create a thread though, just find out what other's have posted (simple google search should work too).

I always like to start off troubleshooting from the easiest > hardest. Assuming some post success with simple O2 sensor swap, I would start there. In general O2 sensor is pretty easy to change assuming it's not in the PITA spot (which might apply to your car, I don't know). You can rent a special socket from Autozone for free...

I'm pretty sure you can easily check if it's the cat by simply unscrewing the O2 sensor and letting it hang (bypassing any Cat reading). Plug the open hole with a washer/bolt of the same size.

At that mileage I wouldn't probably bother with Cat swap. Assuming you have success with O2 sensor removal/plugging......leave it that way and clear the code.

Unless the research tells you otherwise, start with the post Cat sensor.

I had a similar issue when I put a test pipe on my passat. I was getting O2 sensor/cat codes.I removed the post cat O2 sensor all together and plugged the hole with a bolt. It did the trick for me.

is it really that expensive? in college i had a nissan that a hole rusted into the cat . had it replaced with some aftermarket unit and with labor it was barely over $200.

 

[jayram1408]

By temperature?

I was watching some videos on Youtube (learning how to replace a knock sensor, yay!), and happened across a video ( <Link! ) claiming that you can find out if your catalytic converter is bad by checking the temperature (using an IR thermometer) of the cat (close to the O2 sensor) and the pipe before (close to the engine) and after the cat. Video says pre-cat should be ~300F,a good cat should be ~500F.


If you've been reading my previous threads, you'll know why this interests me. And it's not only because I'm a total car noob. If you haven't: The reason this interests me is because I have a P0420 (cat efficiency below threshold, yay!), which from my reading says that could either mean the cat(s) is/are bad, or one or both O2 sensors could be bad. The vehicle ('02 Impreza 2.5TS, almost 208K miles) was recently in the shop for some unrelated repairs, and the shop said it's for sure the cats, but I'm not sure how they came to that conclusion, since we didn't ask them to do anything related, and didn't pay them for any diagnostics.

So, after a quick drive to the store and back (about 4 miles round trip, mostly highway), I grabbed the trusty Harbor Freight IR Thermometer, crawled under the car, and checked... Closest spot I can reach to the engine was ~350-400F, closest I could get to the O2 sensor was ~550F, post-cat was ~350-400F.

Anyone have any knowledge/experience with these sorts of things? I'd really rather my O2 sensors were bad than replace one or both cats, since I can get both sensors for about the same as one cat. lol

If the catalytic convertor runs too long with high temps, eventually it will cause the catalyst material to no longer function as designed. Thus lower temp.

 

[BoomerD]

Thank you for bumping an almost 10 year old thread. We FINALLY get the answer...