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Old 02-01-2016, 09:48 PM   #1
Chrs92
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Default Cat problem

Hey guys I'm new to this site and wondered if anyone could give me some info as to why my cat converter won't "light off". I have an 03 wrx, my first issue was the backfire and I read that the BOV cause your car to run rich therefore backfire so I replaced that for the stock BPV. No more backfire now but the cat still will not light off, I don't have a check engine light on or any codes, all it tells me is the cat isn't lighting off. Any ideas?
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Old 02-01-2016, 10:15 PM   #2
Loyale93
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What the hell is "light off" ?
What's the actual problem you're experiencing?
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Old 02-01-2016, 11:12 PM   #3
Chrs92
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Quote:
Originally Posted by Loyale93 View Post
What the hell is "light off" ?
What's the actual problem you're experiencing?
I believe it's when the cat gets hot enough to burn the remaining gasses.
That's my problem. I can't pass smog.
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Old 02-02-2016, 12:44 AM   #4
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Quote:
Originally Posted by Chrs92 View Post
I don't have a check engine light on or any codes, all it tells me is the cat isn't lighting off. ?
I'm a bit confused. If it has no CEL, how is the car telling you the car won't "light off"? Does your car talk to you or something?



On a serious note, is the exhaust all OEM or have parts been replaced?

Also, maybe you should do some quick searching to understand what a catalytic converter is and how it works. The temperatures of your exhaust will remain constant, so if your catalytic converter is failing to do its job, it has likely broken apart or clogged up. It's not like there is some sort of heater connected to it that heats it up to burn off combustion byproducts...

Last edited by ImprezaLover94; 02-02-2016 at 12:51 AM.
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Old 02-02-2016, 01:18 AM   #5
Chrs92
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I have a scanner and plugged it in. When I search for codes it doesn't shoot me any nore is there a check engine light but when look up how the rest of the car is running on the scanner that's where it shows me the cat converter isn't working right. I find that ridiculous because it's a brand new cat and the scanner is telling me the same thing as it did with the old cat. Everything is stock at this point on the car.
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Old 02-02-2016, 08:03 AM   #6
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Light off is exactly Chrs92 said, it's when the catalyst is hot enough to actually catalyze any un-burnt hydrocarbons (fuel).

There are a couple ways to know that light off has occurred. One is pre and post catalyst temperatures. If they are all the same, despite the engine running rich, the cat isn't doing anything. The second is with the rear O2 sensor. By varying the mixture between rich and lean, the rear (or downstream) O2 sensor can detect catalyst efficiency.

I don't recall the location of the EGT sensor in the exhaust on the 03, but you might want to check it. Data log the EGT, and make sure it gets hot when you step on the throttle. If it doesn't, you need to replace it. Find out if there is a pre or post cat EGT and check those. If they are cheap, just replace them.

Second is the downstream O2. I would expect a code if that's the problem, but Subaru didn't have codes for the MAF not working, so who knows.

Finally, is the scanner telling you light off isn't occurring, or do you have an OBD code? If you have an aftermarket scanner and it's telling you this, I would read the directions of the scanner to see exactly what it means. Is it a P code? It could be without setting the check engine light. BUT, i'm very skeptical, since the EPA requires notification if any emissions related component isn't working. Is your check engine light burned out?
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Old 02-02-2016, 09:00 AM   #7
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How long have you waited for the code to go away?
any time I've replaced a cat it takes almost a week for it to go away.
You need a certain amount of full drive cycles.
This is to prevent people from just un hooking their battery before inspection to clear codes.
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Old 02-02-2016, 01:27 PM   #8
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Quote:
Originally Posted by Chrs92 View Post
I find that ridiculous because it's a brand new cat and the scanner is telling me the same thing as it did with the old cat. Everything is stock at this point on the car.
So if you put a brand new catalytic converter on, how is the car all stock? Did you replace an entire section of exhaust with a new OEM section? Or did you cut a piece of exhaust and weld in a new catalytic converter (that isn't OEM)?



Quote:
Originally Posted by Bacon117 View Post
If they are all the same, despite the engine running rich, the cat isn't doing anything.
I'm a bit confused by this part. A lean engine runs hotter, which creates higher exhaust temperatures and allows the catalytic converter to do its job. Why would the engine run rich if they are the same before and after the cat? Wouldn't it run leaner to try to burn off the byproducts?



Quote:
Originally Posted by Bacon117 View Post
I don't recall the location of the EGT sensor in the exhaust on the 03, but you might want to check it. Data log the EGT, and make sure it gets hot when you step on the throttle. If it doesn't, you need to replace it. Find out if there is a pre or post cat EGT and check those. If they are cheap, just replace them.

Second is the downstream O2. I would expect a code if that's the problem, but Subaru didn't have codes for the MAF not working, so who knows.

Finally, is the scanner telling you light off isn't occurring, or do you have an OBD code? If you have an aftermarket scanner and it's telling you this, I would read the directions of the scanner to see exactly what it means. Is it a P code? It could be without setting the check engine light. BUT, i'm very skeptical, since the EPA requires notification if any emissions related component isn't working. Is your check engine light burned out?
The EGT on a 2003 WRX is right after (in terms of exhaust flow) the first catalytic converter, in the upipe. Just curious, why would he need to replace the EGT if it wasn't reading correctly? Does the engine change fueling based on exhaust temperature or does it only change based on the 2 o2 readings? I'm asking because most people replace their uppipe with one without a catalytic converter (mainly to prevent it from breaking up and destroying the turbo) and most aftermarket uppipes don't have an EGT bung, yet they still seem to pass emissions if catalytic converters are used downstream from the turbo.

Here is a diagram of our exhaust systems:




Also, I don't think his CEL is burnt out because he hooked up a code scanner and scanned but didn't find anything. So even if the light itself wasn't working, the code scanner would pick up codes that the ECU was picking up.

But yeah I am also curious. How is the code scanner telling you that that catalytic converter isn't working? What code/message is it showing?
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Old 02-02-2016, 01:32 PM   #9
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Maybe he is talking about the readiness monitor?
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Old 02-02-2016, 01:35 PM   #10
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Quote:
Originally Posted by ImprezaLover94 View Post
So if you put a brand new catalytic converter on, how is the car all stock? Did you replace an entire section of exhaust with a new OEM section? Or did you cut a piece of exhaust and weld in a new catalytic converter (that isn't OEM)?





I'm a bit confused by this part. A lean engine runs hotter, which creates higher exhaust temperatures and allows the catalytic converter to do its job. Why would the engine run rich if they are the same before and after the cat? Wouldn't it run leaner to try to burn off the byproducts?





The EGT on a 2003 WRX is right after (in terms of exhaust flow) the first catalytic converter, in the upipe. Just curious, why would he need to replace the EGT if it wasn't reading correctly? Does the engine change fueling based on exhaust temperature or does it only change based on the 2 o2 readings? I'm asking because most people replace their uppipe with one without a catalytic converter (mainly to prevent it from breaking up and destroying the turbo) and most aftermarket uppipes don't have an EGT bung, yet they still seem to pass emissions if catalytic converters are used downstream from the turbo.

Here is a diagram of our exhaust systems:




Also, I don't think his CEL is burnt out because he hooked up a code scanner and scanned but didn't find anything. So even if the light itself wasn't working, the code scanner would pick up codes that the ECU was picking up.

But yeah I am also curious. How is the code scanner telling you that that catalytic converter isn't working? What code/message is it showing?
So, your last question about how the scanner is telling him it isn't working feeds into the response about the EGT sensors. There are only a couple ways to measure if the catalyst is working. One is the response of the downstream O2, and the other is with the EGT sensor. No, I don't know how Subaru does this, but I do know how catalysts work. You can model the response of the EGT with a working CAT and a dead CAT, then compare them. This comparison is what the ECM can use to determine if the CAT has lit off during a cold start.

So, while I don't know how Subaru's ECM does it exactly, there are only two sensors that go into determining if a cat is working. O2 sensors and EGT sensors.

I think the better question would be to find out more information about WHY it thinks the cat has not lit off. From there, it's probably best to follow the pinpoint tests in the FSM.
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Old 02-02-2016, 01:42 PM   #11
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Quote:
Originally Posted by ImprezaLover94 View Post
I'm a bit confused by this part. A lean engine runs hotter, which creates higher exhaust temperatures and allows the catalytic converter to do its job. Why would the engine run rich if they are the same before and after the cat? Wouldn't it run leaner to try to burn off the byproducts?
When an engine runs rich, there are extra hydrocarbons exiting the cylinder. (Hydrocarbon is just a fancy word for fuel). Since we don't want this in the atmosphere, the catalyst literally burns the extra fuel. The exhaust coming out of the cat will be hotter than the exhaust coming into the cat if it's running rich. So, with knowing that it gets hotter, we can tell if the cat is working. Either by a mathematical model or by using two EGT sensors, one before and one after the cat.

It seems backwards, but a catalyst doesn't work in a lean burn condition, since it's job is to burn left over hydrocarbons (fuel), and if you are lean, there is nothing to burn.
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Old 02-02-2016, 04:12 PM   #12
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CAT's 'catalyze' the reaction between oxygen and unburnt fuel leaving your motor. A motor that is messed up and running extra rich will end up with a CAT that gets SUPER hot. They are not meant to handle tons of fuel, they are meant to burn the tiny bit of leftovers. Ever stand near a pre 1985 car while it runs? That slight fuel smell is what the CAT handles.
A lean burn is hotter yes, but that has nothing to do with "allowing a cat to do it's job"
A CAT needs to get up to a certain temperature to function. . . I guess this is what people call "lighting off."
ECU's do not determine if the cat is working by monitoring stupid exhaust gas temp, that is bizarre. The O2 sensors tell the ecu if there is a catalyst problem.

If it isn't "lighting off" or whatever you want to call it that means it doesn't work.

A PERFECTLY running engine will have fuel smell in the exhaust with no CAT, or a bad one. So does the exhaust smell of fuel or not?
If not it is working perfectly and you have some code issue.
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Old 02-02-2016, 04:35 PM   #13
Chrs92
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Alright, well to start off my check engine light isn't burnt out. I see the damn thing every time ignition switch is switched before I turn car on so that's not an issue. Second, my car at one point was giving me misfire codes in a couple of my cylinders so I changed my spark plugs and voila, check engine went away although the readings from the post o2 sensor was still telling me the CAT wasn't working right so i figured the misfiring messed up my CAT so i got a new one and put it on, still didn't change a thing. Then that's when I started doing more HW online as to why my CAT wasn't "lighting off" and alot said that if the car runs too rich then that will affect it and a BOV could be the reason it's running rich, do I took my BOV off and slapped the stock BPV on. I took the car out for a 40 mile drive, checked the monitors on the scanner and everything checked out fine, (o2 sensor, evaporative system) everything fine but the catalytic converter. Now there are 2 sensors, the one that's on the CAT and the one that's before the CAT by the up pipe. Some of you seem to call it the EGT but I know it as the fuel/air sensor. My next step is checking the fuel air sensor and going from there.
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Old 02-02-2016, 06:08 PM   #14
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EGT and air fuel are two seperate sensors. your car has both.

Quote:
Originally Posted by Chrs92 View Post
everything fine but the catalytic converter.
How exactly do you know this? What EXACTLY did the scanner say, what code and what message?
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Old 02-02-2016, 06:29 PM   #15
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He's saying that he has no code but that he is *somehow* reading the 2 oxygen sensors and they read the same, which would mean the cat isn't doing its job. Normally, we'd expect a CEL, right? Has this car been previously tuned by someone else and the codes turned off because they were running a catless downpipe?

If the oxygen sensors are old, replace them. Beyond that, perhaps a tuner could figure out what's going on.
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Old 02-02-2016, 07:51 PM   #16
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That's why I'm asking exactly what he means. When the cat efficiency is calculated, the PCM is looking at real time, high speed data. I'm not sure you can data log through the OBD port and be able to tell that the catalyst is working efficiently. I'm not saying you cannot, but OBD port CAN messages are low priority, and may not be fast enough.

So, again, is it a code, or are you judging it's not working by data logging, or are you judging it's not working based on tail pipe sniffers at a shop?
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Old 02-02-2016, 07:56 PM   #17
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If you are curious, this is kind of similar to what the PCM sees. It puts a step input into the system and reads the output. So in this graph, the bottom is like the upstream O2, and the top is the downstream O2. The transportation delay, time constant (rate of rise) and other factors are measured, then put through a mathematical model to calculate efficiency. (The graph is for illustration purposes only, it's not real O2 sensor data.)

This stuff is really interesting, which is why I'm studying it for my Masters Degree.

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Old 02-16-2016, 09:22 PM   #18
Chrs92
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Default No problemo

It's all good boys. Fixed the problem. I went to pick up some parts for the ol Rex and ended up picking up an old Lexus o2 sensor for FREE! Slapped it on and ran the hell out of it in the canyons and VOILA. Finally passed smog.
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