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Ask Daryl Your Own Question
Daryl, Shop Owner, Master Mechanic
Category: Cadillac
Satisfied Customers: 12549
Experience:  28 years experience, Master Mechanic
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Cadillac STS: P0420 bank1 cat problem

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P0420 bank1 cat problem
Hello and thank you for trusting me with your question. My name is XXXXX XXXXX I will do my best to help you with your problem. With that code the shop you are referring to is correct, the cause is typically the Cat. The other 2% that code is usually caused by the oxygen sensors. Let me know if you have more questions on this I am always willing to help, Thanks.
Customer: replied 4 years ago.

I intend on doing this repair myself so the guidance I need is with the replacement of the cat. I understand it is on the passenger side and located as the lower cat. That is to say that it is not the pre cat but rather follows the exhaust to about the middle of the car. Can you help me with this?
I just looked at a diagram for this exhaust system and it is only showing a right and left Cat, they just bolt on the upper and lower and there is a oxygen sensor that needs to be removed so I really am not sure what kind of help you are looking for?
Customer: replied 4 years ago.

I was looking for the diagram because I haven't been under the car yet. Also I was looking for you to verify that it is the passenger side for me based on the code. I would also be looking for a replacement cat so a part number would help me. Can you help me with these things and any tips on replacing the part like special tools needed or heat applied for removal?

Yes that Cat is the passenger side, more than likely you will need some heat but in some cases you get lucky. This is a pretty tight job as in all the parts are in tight quarters, you are going to need a set of sockets and long extensions and wobbles to get in tight angles. Below is a diagram,


It appears the diagram is not working, give me a minute.


Customer: replied 4 years ago.

OK I got it through the link. It looks pretty simple to do. Should I replace both or just the one they said is not good?

I would just do the one, the other one may last the life of the car but you never know for sure so maybe down the road in a few years it may need replacing.
Customer: replied 4 years ago.

Ok thanks. How about a part number for the cat replacement? Will I need to purchase anything other than the cat like gaskets or lubricant for the threads on the O2 sensors?

Are you thinking you are going to use after market parts or factory/dealer parts?
Customer: replied 4 years ago.

That was my next question. I would like to use the dealer part but I drive a lot and this car has 221900 miles on it. If aftermarket performance is ok to use than I would go that way to save the money. A suggestion of an aftermarket part would be helpful unless you think all aftermarkets are relatively equal.

I personally would stick with the factory part for this the after market is kind of a toss up to weather or not the performance will be good enough, if it is not the check engine light will come back on. I can not get part numbers for dealer parts at this time of the day but if you take your VIN number with you the dealer will not have a hard time getting you the part you need. As for gaskets I believe that you need one on the top but the bottom you do not need any, these are cone type fittings, I usually put a anti seize on the threads of the oxygen sensors, just a little.
Daryl and other Cadillac Specialists are ready to help you
Customer: replied 4 years ago.

Daryl, can you clarify the location of this cat again for me. I once replaced a cat on the drivers side in my driveway on my back that was a Bear. If I remember right, I removed a heat shield then discovered the cat reached the manifold and was not easy to maneuver around in these tight spots. Is this passengers cat in the same location just on the passengers side or is it another destination of cats further down the exhaust system toward the back of the car? I was thinking it is around the middle of the car and second to one closest to the manifold? Not opposite of the one i had replaced a year ago.

The one closest to the manifold is the one that would be bad on the passenger side, there is no other one further down the exhaust I believe what you may be referring to is a resonator its a kind of a pre-muffler to make the system more quite.
Customer: replied 4 years ago.

Thanks I was thinking that the resonator was what I needed to change based on the mechanics description. So now I know that this job is not as simple as a previously stated. However, I did do the drivers side myself last year. Do you think the passengers side will be more difficult for me to do? Also I have been looking online for replacement cats and came across re-certified OEM cats. What do you think about these. I have never heard of these before.

I have never heard of the re-certified cats either, I think I would pass on that. I would expect the drivers side and the passenger side to be about the same as far as difficulty.