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2003 Cadillac Deville Trouble code P0420

Tiny
JAREDR123
  • MEMBER
  • 2003 CADILLAC DEVILLE
  • 4.6L
  • V8
  • FWD
  • AUTOMATIC
  • 122,000 MILES
I have an 03' Cadillac Deville Base Sedan and it is throwing the code p0420 (Catalyst System Low Efficiency Bank 1). This code just came up today. Right now it is the only code showing. Just two weeks ago I had codes p0101 and p0410. I installed a brand new GM Mass Air Flow Sensor and a brand new GM Secondary Air Pump. And both of those codes went away. But now two weeks later, after about 400 miles the code p0420 came up. My cars power is not where it should be at. I will floor the gas pedal and the Rpms are slower to get up and accelertion is sluggish. It will increase speed decently fast but I don't think as fast as it could. I can reach speed to over 100mph. And its been doing this since I got it about a year ago. So I dont know if the CAT is completely clogged but maybe a little im guessing. And when I floor it, itll give these couple little hesitations around 35mph and then itll take off faster than it usually does after that. So I am just wondering if I need a whole new CAT or a new O2 sensor. And If I do need a new CAT, would it be bad if I got an aftermarket one or should I get the GM one. I know GM CAT is way more expensive but if you strongly suggest to get a GM one I will do that. So just wondering if I need a new CAT or maybe an 02 sensor or even something else.
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Monday, April 27th, 2015 AT 10:47 AM

15 Replies

Tiny
ED-WEEZY21
  • MECHANIC
  • 248 POSTS
Unfortunately, you're going to need a CAT. I see these codes all day at work and nothing fixed then other than replacing it. I always suggest going OEM because you can almost guarantee that you won't have a problem because they are made with the higher quality precious metals which is why it's more expensive but I would check all our options out first for aftermarket CATs and read reviews. I've never had a problem with the Walker brand when I went aftermarket and it's a quarter of the price. Usually the CAT goes bad due to a cause but it looks like you fixed the cause by replacing the sensors. Good luck.
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Monday, April 27th, 2015 AT 12:23 PM
Tiny
JAREDR123
  • MEMBER
  • 29 POSTS
Ok thanks for the information. I just called my mechanic and he said for the GM CAT would cost over $1600 for part alone. I would rather put GM CAT in but right now I am a college student and don't really want to pay for that right now. So We talked about a Walker direct fit CAT which costs about $360 for try part. So it would save me $1300. Do you think the Walker CAT will be just as efficient or not fail me so I have to buy another one? And the weird thing is right now is that I get excellent gas mileage on highway and even in town for this car. About 28 on highway and 20-22 in town. Is that normal for a partially clogged CAT?
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Monday, April 27th, 2015 AT 12:37 PM
Tiny
ED-WEEZY21
  • MECHANIC
  • 248 POSTS
It could possibly cause problems depending on how bad it's clogged. Since it sounded like you had an air intake problem, too much fuel was being dumped in and not burned properly causing the CAT to overwork. The CAT is made up of a honeycomb material that stores oxygen and burns off any unburned fuel. It may cause back pressure which would lead to engine problems. For the Walker brand, check on their warranty. It's worth a shot to save $1300 and if the code does return, maybe the parts store will let you exchange it.
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Monday, April 27th, 2015 AT 1:12 PM
Tiny
JAREDR123
  • MEMBER
  • 29 POSTS
Ok and what were you talking about that could possible cause problems? And I think I am going to go with the direct fit Walker CAT. I mean the car drives really good and idles good. The only time I feel anything that's wrong is when I go hard on the gas. If in driving normal I don't feel any hesitation or anything. If I just go normal, you wouldn't be able to tell anything I'd wrong with it. And if I replace the CAT, I also replace the O2 sensors right? And I do use ethonal free gas ever since I got the car (about a year ago) so that's a plus. When I go hard on the gas, which isn't often, I just do it once in awhile to see how the CAT is working because I have been having troubles with it. So when I go hard on the gas from a stop, I can get rpms up pretty quick around 4500. But it's just when I'm driving say around 35mph and go hard on the gas, it's very hard to get rpms up. Speed does increase just not as fast as I think this car can go.
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Monday, April 27th, 2015 AT 2:56 PM
Tiny
JAREDR123
  • MEMBER
  • 29 POSTS
And do you think this is something I should fix right away or wait. What are possible outcomes if I wait to replace the CAT.
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Monday, April 27th, 2015 AT 2:59 PM
Tiny
ED-WEEZY21
  • MECHANIC
  • 248 POSTS
I was just mentioning that any part of an exhaust that gets clogged or restricted can start causing problems like hard starts or no starts, hesitations and other things along those lines. But it's rare. I've only seen it once during the winter where the mid pipe had water in it and it froze inside of it causing a no start because the pressure couldn't be released. Regarding fixing it right away, an improper working CAT is mainly affecting the environment with pollution with more NOx and hydrocarbons. Also depending on which state you reside in, laws may be different when it comes to vehicle inspections. My state of Mass requires a safety and an emissions test every year for vehicles 2001 and up and would fail the emissions test with a check engine light.
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Monday, April 27th, 2015 AT 3:55 PM
Tiny
JAREDR123
  • MEMBER
  • 29 POSTS
But if I got a new CAT my performence issues more specially the acceleration one and also not make my engine overheat due to clogged CAT.
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Monday, April 27th, 2015 AT 4:22 PM
Tiny
ED-WEEZY21
  • MECHANIC
  • 248 POSTS
I can't guarantee that it will fix the acceleration problem that you say you're having. It could be that or you could have another problem on your hands. A clogged CAT wouldn't cause an engine to overheat. If anything, it would cause the car to choke out and shut off if it's that bad but yours doesn't seem that bad considering you're saying that it drives fine almost every time.
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Monday, April 27th, 2015 AT 4:30 PM
Tiny
JAREDR123
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  • 29 POSTS
But doesn't a clogged CAT decrease performance? And I have the trouble code p0420 pointing to the catalytic converter.
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Monday, April 27th, 2015 AT 4:35 PM
Tiny
ED-WEEZY21
  • MECHANIC
  • 248 POSTS
Yes, a restricted exhaust will cause performance issues but the code P0420 isn't set due to what you are experiencing. P0420 is a below threshold code which means that the rear oxygen sensor isn't reading the proper output from the CAT. The upstream oxygen sensor will move from lean to rich going from.9 volts for rich condition and.1 for lean condition. The upstream working properly changes up and down before going into the CAT. The downstream on the other hand monitors what's coming out of the CAT. If the CAT is working properly, the downstream oxygen sensor should stay around.45 volts and have very little to no change indicating a proper reading. If the downstream oxygen sensor starts to look like the upstream sensor, this indicates that emissions are not being controlled properly and THATS what sets the check engine light. I hope that helps you understand more how the system works.
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Monday, April 27th, 2015 AT 5:20 PM
Tiny
JAREDR123
  • MEMBER
  • 29 POSTS
So my question is, should I replace the CAT or 02 sensor or anything else to fix the p0420 code? And what is causing this acceleration issue?
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Monday, April 27th, 2015 AT 5:24 PM
Tiny
ED-WEEZY21
  • MECHANIC
  • 248 POSTS
The downstream sensor isn't defective. It's properly detecting that the CAT isn't working so I would replace the CAT. But while you're at it, oxygen sensors are maintenance items and if they've never been changed, I would go ahead and replace those as well. This should improve gas mileage and engine performance. The acceleration problem can be various problems but I would start by checking the transmission fluid condition. If it's never been changed, I would do a transmission service. If the transmission filter is replaceable, I would change that as well.
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Monday, April 27th, 2015 AT 5:33 PM
Tiny
JAREDR123
  • MEMBER
  • 29 POSTS
Alright thank you I will let me mechanic know!
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Monday, April 27th, 2015 AT 5:41 PM
Tiny
JAREDR123
  • MEMBER
  • 29 POSTS
But my gas mileage is really good for my car so that's not an issue
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Monday, April 27th, 2015 AT 5:41 PM
Tiny
ED-WEEZY21
  • MECHANIC
  • 248 POSTS
It's just an added bonus if it improves your gas mileage even more haha. I just say this because after awhile, oxygen sensors start to become sluggish and they don't change from rich to lean as quickly even though it may still be within specification. That sluggishness causes a slower response to the computer to control the fuel. So in theory, a faster response causes the computer to react faster on how much fuel to control. Hope all this helps and good luck.
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Monday, April 27th, 2015 AT 6:00 PM

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