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Old 11-18-2012, 02:47 PM   #1
Tucci139
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Rally Stuff o2 sensor replacement with catalytic convertor?

Replacing exhaust on my 2005 Subaru Impreza RS with a Cat back system and a OEM replacement catalytic convertor (Eastern). The car has 60,000 miles on it and the mechanic who did the inspection said it had a bad cat as well as needs new water pump ,etc. He is a good and honest mechanic I know for awhile.

My question is, when I replace cat should I just go ahead and replace upstream and downstream O2 sensors at the same time since I will be replacing everything? Or, is this a waste of money and I should just keep using the same sensors. Be interested to here others opinions about this.

Most postings I read on here are when people get the CEL codes and then they have to fix it. Just thought it maybe best to do preventative maintenance. Also, I will be rallying car a bit and need it reliable and running efficiently.
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Old 11-18-2012, 03:48 PM   #2
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Personally I wouldn't bother to get new sensors unless you can get a deal on them. It doesn't really save you a bunch on labor, if you need to replace one later. And atleast on my car the cheapest I could get both for was over $200, And that was on amazon, non-universal type.
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Old 11-19-2012, 02:47 PM   #3
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I like to take them out and burn the carbon off every once in a blue moon. Just grab a propane torch, heat the tip up till it's close to glowing, and repeat all over the sensor tip. The carbon comes right off and they seem to respond quicker again.

Eventually though it will be a dead sensor and require replacement.

I'd also try seafoaming the car to see if the cat can be "cleaned" that way. Unless the catalyst itself is majorly damaged or partially melted you can usually get them clean enough to work again.
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Old 11-19-2012, 03:05 PM   #4
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OP, just replace the cat and forget about the sensor, if it's not giving you issues then why spend the money. Its a quick 2 min job when the time comes anyway.

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Originally Posted by gdoggmoney View Post
I like to take them out and burn the carbon off every once in a blue moon. Just grab a propane torch, heat the tip up till it's close to glowing, and repeat all over the sensor tip. The carbon comes right off and they seem to respond quicker again.
Neat trick, I'll have to try that.
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Old 11-19-2012, 03:47 PM   #5
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OP, just replace the cat and forget about the sensor, if it's not giving you issues then why spend the money. Its a quick 2 min job when the time comes anyway.



Neat trick, I'll have to try that.
I suppose if you get them too hot for too long you will damage them. But so far so good here. I had a bad pre O2 exhaust leak and both sensors were extremely sooty. Burning them clean the car ran much better no changes (did it a day ago.) Yesterday night I went around doing logging and scaled my maf, even better than before.
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Old 11-19-2012, 03:52 PM   #6
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That's exactly what I needed to hear
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Old 11-19-2012, 09:25 PM   #7
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Great ideas thanks - I decided to wait on O2's as suggested. May even wait on cats for now. The mechanic says a metal shaking sound when I start the car is the cat. it goes away quickly - I thought maybe it is the heatshield or something - so I will wait until I see a drop in performance from a clogged cat or something. Any other signs to verify the cats are bad?
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Old 11-20-2012, 02:02 AM   #8
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DTC P0420 and a check engine light accompany a bad kitty, but can still be downstream O2 sensor. if you are not concerened with emissions and just want to pass you can always use a "spark plug anti fouler" drilled out to use as a spacer on the O2 sensor, rather than replacement, if the converter is bad. Costs $5...

And as far as reliability and efficiency go, a bad kitty (or downstream O2 sensor for that matter) only effect would be increased HC, CO, and NOx emissions. It would have no impact otherwise on the car.

Does your mechanic have any other reasons for thinking the cat is bad? heatshields make all kinds of rattling noises and are easy to fix... I would never even think about replacing a convertor as "preventative maintenance".

If the water pump is bad, go ahead and do a timing belt and a tensioner at the same time. It should only cost you the parts no extra labor.

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Old 11-20-2012, 09:03 AM   #9
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If that cat is clogged up or partially melted, it can cause even more exhaust obstruction and raise EGT's to exhaust valve greying/cooking levels. The heat shielding on them is easily removed with a cut of wheel and swearing.
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Old 11-20-2012, 11:36 AM   #10
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Quote:
Originally Posted by ProdriveDreams View Post
DTC P0420 and a check engine light accompany a bad kitty, but can still be downstream O2 sensor. if you are not concerened with emissions and just want to pass you can always use a "spark plug anti fouler" drilled out to use as a spacer on the O2 sensor, rather than replacement, if the converter is bad. Costs $5...
I suggested that in another thread for a 2005 Impreza with the dreaded P0420 code. Apparently this trick on the newer cars(2005 and up) can cause some issues with the emissions system , on older cars it works like a charm though.
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Old 11-20-2012, 11:52 AM   #11
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When I read through OP it doesn't sound like a bad cat at all. So, you don't have a CEL just a rattle while cold? That's like the official noise of Subaru heat shielding.

My advice. Go to dealer if you are worried. If your cat is bad it should still be under the 80,000 mile emissions warranty and be free.

As for water pump why do you need a new one of those? I'm guessing this mechanic not very knowledgeable on Subaru?
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Old 11-20-2012, 12:05 PM   #12
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Quote:
Originally Posted by gdoggmoney View Post
If that cat is clogged up or partially melted, it can cause even more exhaust obstruction and raise EGT's to exhaust valve greying/cooking levels. The heat shielding on them is easily removed with a cut of wheel and swearing.
way easier method... buy a box of self drilling screws. find where heat shield is loose. Put a screw in so that it goes through the shield and then pulls it tight away from the pipe. DO NOT KEEP GOING AND DRILL THROUGH YOUR EXHAUST... (Duh... its actually hard to do considering the shield is like 1/32 and pipe is 1/8. But I figured some dummy would do it.)

Where the heat shield is rattling against itself, grab a big worm drive pipe clamp from the hardware store and slip over that section, then tighten down like crazy. When I worked at a Subaru dealer that was what we did to alleviate customer complaints of heat shield rattles. Fast, easy, and leaves the shield intact.

And its swear free. which depending on your disposition may not be a feature.

also a cat clogged that bad would not be symptom free... you would know something was wrong.
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Old 11-20-2012, 12:53 PM   #13
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^^ good call.
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Old 11-22-2012, 12:04 AM   #14
Tucci139
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Quote:
Originally Posted by pieces View Post
When I read through OP it doesn't sound like a bad cat at all. So, you don't have a CEL just a rattle while cold? That's like the official noise of Subaru heat shielding.

My advice. Go to dealer if you are worried. If your cat is bad it should still be under the 80,000 mile emissions warranty and be free.

As for water pump why do you need a new one of those? I'm guessing this mechanic not very knowledgeable on Subaru?
Correct on all accounts - seems like foreign cars aren't his speciality. I still think he is honest, just was mislead by noises. I checked with another shop in town that preps race cars - mostly import. Thanks for the suggestions - no cat or O2 sensors for now.
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Old 11-22-2012, 12:07 AM   #15
Tucci139
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Quote:
Originally Posted by ProdriveDreams View Post
way easier method... buy a box of self drilling screws. find where heat shield is loose. Put a screw in so that it goes through the shield and then pulls it tight away from the pipe. DO NOT KEEP GOING AND DRILL THROUGH YOUR EXHAUST... (Duh... its actually hard to do considering the shield is like 1/32 and pipe is 1/8. But I figured some dummy would do it.)

Where the heat shield is rattling against itself, grab a big worm drive pipe clamp from the hardware store and slip over that section, then tighten down like crazy. When I worked at a Subaru dealer that was what we did to alleviate customer complaints of heat shield rattles. Fast, easy, and leaves the shield intact.

And its swear free. which depending on your disposition may not be a feature.

also a cat clogged that bad would not be symptom free... you would know something was wrong.
More great advice thanks.
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