If my Catalytic converter were shot, would it show up on an OBDII scan?

6716

Original poster
Member
Jul 24, 2012
830
So here's the deal: I drove on a bad thermostat for like ... a while. Don't really want to admit publicly how long I ignored the CEL. I got the PO120 code, but the truck fired up and drove around so I rolled with it. Didn't make it a priority until I understood what it could do to the cat, at which point I finally made the repair.

But it was "a while," and I figured the cat was fried, and I figured I would not pass emissions testing (Wisconsin).

But today I passed emissions, which has recently been privatized and would seem to be not much more than an OBDII scan at the shop down the street.

What do you think? Am I good to go, or am I both lucky and fooling myself?

I don't think I have power issues, and I was getting 20-21mpg on a recent road trip.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
As long as you have no CEL, pending or didn't just clear a light before the test, you will pass in Wisconsin.
A plugged cat will show-up as a loss of power when really getting on the gas.
You may have indeed dodged a bullet.

However, a P0120 is for a TPS, not a t-stat. Did you mean a P0128?
 

dingle

Member
May 26, 2012
59
If the cat is bad it will throw a code and it will be a p0420. Thats been the case with all the ones that I've had go bad. Doesn't mean it can't go bad without throwing a code though
 

6716

Original poster
Member
Jul 24, 2012
830
Wooluf1952 said:
However, a P0120 is for a TPS, not a t-stat. Did you mean a P0128?

Yes, actually, should have been the 128.

So, without codes, and without noticing a lack of power ... how would I confirm (or not) the status of the converter?
 

Boricua SS

Member
Nov 20, 2011
3,080
Ohio
im not sure about our platform (GMT360's), but my DD grand prix gt did not throw a code when my converter went out (was clogged)... there was a HHUUUUUUGE loss in power and it actually started messing with my transmission, making it slip and miss shifts because rpm's did not match mph...

so if there arent any codes, you passed inspection, and are still getting 20+ mpg's, i think you just answered your own question and you shouldnt worry about the converter anymore...
 

tbuckalew14

Member
Nov 20, 2011
380
Boricua SS said:
im not sure about our platform (GMT360's), but my DD grand prix gt did not throw a code when my converter went out (was clogged)...

My wifes jeep also did not throw a code. Sometimes it rode fine..others it would stall when you came to a stop, or feel like it was about to stall. Replaced the cat a few months ago, along with a new o2 sensor, and have had no issues.
 

6716

Original poster
Member
Jul 24, 2012
830
tbuckalew14 said:
My wifes jeep also did not throw a code. Sometimes it rode fine..others it would stall when you came to a stop, or feel like it was about to stall. Replaced the cat a few months ago, along with a new o2 sensor, and have had no issues.

What made you decide to replace the cat to solve the problem?
 

Boricua SS

Member
Nov 20, 2011
3,080
Ohio
6716 said:
What made you decide to replace the cat to solve the problem?

because the dealer told me too :crackup:

they or any competent mechanic will know the symptoms of a failing cat... the tech took my cat for a ride and immediately came back with prices on cats... an hour later, i had a new car and 02 sensor, and all has been fine since..
 

Chickenhawk

Member
Dec 6, 2011
782
It is important to note that a cat can be bad two ways:
#1 - it can stop cleaning the exhaust properly, or
#2 - it can still clean the exhaust but be plugged, leading to loss of performance.

There is a basic misconception out there that codes indicate mechanical or performance issues. They don't. Lack of codes does not necessarily mean a mechanically-sound vehicle. OBDII codes indicate emissions-related issues. This is why a bad cat may NEVER show a code. It can be so plugged that it only lets a small amount of exhaust through, but that small exhaust is still clean.

The emissions effectiveness of a cat is monitored by the downstream sensor. When the readings start to match the upstream sensor readings, indicating reduced effectiveness, then it throws a code.

The actual flow performance of a cat is not monitored.

If a cat is plugged, there are some common symptoms that may be displayed:
- loss of performance on the highway, beginning with acceleration at highway speeds and climbing hills;
- loss of mileage;
- abnormally high cat temperatures;
- an overall feel of sluggishness, with no other causes or indications.

While there are ways to diagnose cat performance such as removing the front oxygen sensor or measuring the temperature of the cat, the only proven technical method is to measure your exhaust backpressure with an exhaust backpressure gauge. The front oxygen sensor is removed, the gauge is screwed in to the hole and pressure is measured in PSI at idle and again at 2500 RPM. A good cat should read 0 PSI at idle and under 4 PSI at 2500 RPM.

It s simple, fast and cheap.

You can even do it yourself with a gauge that costs less than $100:
View attachment 22215

If, for some reason, you decide to get your exhaust backpressure tested, all we ask is that you post the two readings in here to add to our database of understanding what a good and not-so-good cat will read. There are always slight variations, and with our platforms, the cat readings also vary by year too.
 

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The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Chickenhawk said:
You can even do it yourself with a gauge that costs less than $100
$100!!!!! You mean some tools cost $100?!?!?!?!?!?! Isn't there some alternative that's like $1.89 that will do the same job and not cost a month's worth of beer? Tools are EXPENSIVE, man. Don't tell me I have to buy a tool like that. :frown:

Magenta is the implied "sarcasm" tag on many forums.
 

6716

Original poster
Member
Jul 24, 2012
830
the roadie said:
$100!!!!! You mean some tools cost $100?!?!?!?!?!?! Isn't there some alternative that's like $1.89 that will do the same job and not cost a month's worth of beer? Tools are EXPENSIVE, man. Don't tell me I have to buy a tool like that. :frown:

Magenta is the implied "sarcasm" tag on many forums.

Actually, it looks like a person could pick up the tool for under $40, the major difference between the cheaper and the more expensive being whether the lines are rubber, or stainless/braided/built to last way longer than I'm going to need for this one (hopefully) use ... well, that and how willing you are to be gouged by certain sites for exactly the same tool.

Shoot, when I replaced the well pressure tank and pressure meter on the well system at the house, the new meter (at the overpriced-but-incredibly-convenient local hardware store) was only $6. The previous one couldn't have been much more expensive, and it lasted for 30 years. Fittings and a short length of hose can't add too much, either.

Sure, if I were running a repair shop, I'd pop for the better lines.

People are funny about price, I definitely agree with you there. I sell home improvements, and over time I have found that people who are inclined to be DIY-ers (such as might frequent this forum, perhaps) will tend to be much more price sensitive than the folks who know that they wouldn't have the first clue how to do it and put more emphasis on quality materials and work. Unless they are clueless and dead broke, in which case I don't do well at all.

Now that I think about it, I bet you could, using the power of the interwebs, assemble a perfectly good back pressure test tool for $10.

Since the cat is important but not top of the priority list at the moment, I might just see about assembling one myself, just to show off how cheap I can be.:biggrin:
 

Chickenhawk

Member
Dec 6, 2011
782
Go for it! There will always be lots of folks looking for the DIY solution. As for me, I was ordering a new scan tool with ABS, stability control and airbag code reading ability and I found one for $100 so I just added the OTC backpressure gauge for fun.

I have two Trailblazers in the family and I am not a mechanic, so the braided steel lines will be better for the way I work. (I take the most pessimistic estimate of the time it would take to accomplish any task underneath my truck and simply triple it. I then keep playing with various bits until there are no more than one or two parts left over and I am bleeding from somewhere; then I know the job is done.)

In all seriousness, I was also hoping some forum members from the Manitoba area might need the use of it someday so I got the better version. Good tools are made to be enjoyed by people who know how to use them, will appreciate them and understand that dings and nicks are the sign that a good tool has been used and loved.

One thing to keep in mind about your DIY. Any ordinary pressure gauge will do. There is nothing special about a backpressure gauge. One could conceivably use a gauge designed to measure cylinder compression BUT a gauge designed to read 0 to 200 PSI is going to be VERY hard to read when we are talking 2, 3 or 4 PSI. This is why the backpressure gauge is actually a special gauge because it goes 0 to 15 PSI, making those subtle readings much easier to determine.

Other than the scale, it is just an ordinary pressure gauge, with some hose and a fitting that will go in your oxygen sensor socket. Long gone are the days that mechanics had to drill holes in your exhaust and then weld them up afterwards.
 

Menthol

Member
Dec 8, 2011
177
Chickenhawk said:
In all seriousness, I was also hoping some forum members from the Manitoba area might need the use of it someday so I got the better version. Good tools are made to be enjoyed by people who know how to use them, will appreciate them and understand that dings and nicks are the sign that a good tool has been used and loved.

Ok, bring it when you;re heading to Toronto :rotfl: I would like to check mine but as Roadie says $100 is too much to pay for a tool!:wink:
 

xj2202009

Member
Mar 27, 2012
105
Hi everyone,

My 06 TB i6,4.2 ext, is coming at the 100k miles, I'm getting about 14-15 mpg, my engine is sluggish, I'm not getting any error codes, I cleaned the throttle body, put sea foam in the tank, sometimes when I'm on the highway I feel something unclogging and the truck takes off like a bat out hell. I have noticed a black coat around the tip of the exhaust. I'm thinking spark plugs I will be changing them anyways since I'm at 100k miles..


am I looking a hair balled CAT?
 

xj2202009

Member
Mar 27, 2012
105
I replaced the spark plugs at about 99k miles, they were pretty worn out, it seems to have helped some but I still get the choking now and then, sometimes it seems at if something gives and the truck is awesome again but short after we are back to the choking, in a different thread I started a error p0984 having to do with the transmission slipping but that came at 100025 five miles so I'm thinking the trainy is due an oil change.
 

jrSS

Member
Dec 4, 2011
3,950
^^ if u do change the tranny fluid....Walmart sells supertech dex. Vi fluid. It is g.m. approved and four dollars cheaper a bottle. Juan told me this info.
 

Boricua SS

Member
Nov 20, 2011
3,080
Ohio

xj2202009

Member
Mar 27, 2012
105
Chickenhawk said:
It is important to note that a cat can be bad two ways:
#1 - it can stop cleaning the exhaust properly, or
#2 - it can still clean the exhaust but be plugged, leading to loss of performance.

There is a basic misconception out there that codes indicate mechanical or performance issues. They don't. Lack of codes does not necessarily mean a mechanically-sound vehicle. OBDII codes indicate emissions-related issues. This is why a bad cat may NEVER show a code. It can be so plugged that it only lets a small amount of exhaust through, but that small exhaust is still clean.

The emissions effectiveness of a cat is monitored by the downstream sensor. When the readings start to match the upstream sensor readings, indicating reduced effectiveness, then it throws a code.

The actual flow performance of a cat is not monitored.

If a cat is plugged, there are some common symptoms that may be displayed:
- loss of performance on the highway, beginning with acceleration at highway speeds and climbing hills;
- loss of mileage;
- abnormally high cat temperatures;
- an overall feel of sluggishness, with no other causes or indications.

While there are ways to diagnose cat performance such as removing the front oxygen sensor or measuring the temperature of the cat, the only proven technical method is to measure your exhaust backpressure with an exhaust backpressure gauge. The front oxygen sensor is removed, the gauge is screwed in to the hole and pressure is measured in PSI at idle and again at 2500 RPM. A good cat should read 0 PSI at idle and under 4 PSI at 2500 RPM.

It s simple, fast and cheap.

You can even do it yourself with a gauge that costs less than $100:
View attachment 8483

If, for some reason, you decide to get your exhaust backpressure tested, all we ask is that you post the two readings in here to add to our database of understanding what a good and not-so-good cat will read. There are always slight variations, and with our platforms, the cat readings also vary by year too.


Chickenhawk you pretty much described my symptoms, I think I'm just gonna put the $100 towards a new catalyst and and forget about the tool, as I'm already at 100k miles. now would any aftermarket cat? FL does not have emissions control basically you have to have something that looks like a cat.
 

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