Check engine turns on, CAT or O2 sensor?

I assume the oxygen sensor codes are for sensor rich. This will make the cat overload and trigger a cat efficiency code. Have you checked the air filter if it is clogged it will cause this issue? If the MAF sensor is off will cause this issue also if you have injectors that are starting to go bad because they don't atomize the fuel correctly. Also, if you have the code numbers it will help. Here is the location of the MAF sensor so you can check it out and replace it if necessary. Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.

Hi Ken,

Glad that you're working with me again on another car...
Under Stored codes, there are two codes:
P0420 catalyst system efficiency below threshold Bank 1.
P0153 O2 sensor link slow response Bank 2 Sensor 1.
Under Pending codes, there are 3 codes:
The 2 above, plus a P0133 for O2 sensor link slow response Bank 1 Sensor 1.
Air filter is new, and I believe the MAF is pretty clean... I used MAF cleaner after all the codes popped up but that didn't solve the problem. Both of those sensors were replaced with sensors (rockauto) but the codes stayed on.
All the wiring and connectors to them look pretty good and tight.
Thank you again.

Hi Yum Mah,

Does the car have an exhaust leak before the primary oxygen sensors? This will cause all the codes you have. let me know.

Hi Ken,

Went under the car yesterday and did a good inspection where the exhaust manifolds connect pre o2 sensors and they appeared pretty good, no sign of leaks.
Something worth mentioning here, I replaced three of the six coil packs as they started to give codes over this last year... all three coil packs on bank one side are new. I haven't had any issues so far with the remaining three coil packs.
Is there a sure way of testing for exhaust leaks? May look good to the eye but who knows if sufficient.
Thank you again.

I would do the other coil packs, they should be replaced as a set. Also, you can have a helper power brake the engine which will make any leaks louder and easier to hear.

Hi Ken,

Sorry it took me so long to get back to you on this pain in the A project. I'm due for emissions inspection at the end of Nov so I need to get the CAT issue resolved or they'll fail me.
I replaced all 4 O2 sensors, poured catalytic converter cleaner in the gas tank and drove few hundred miles. Took the pre CAT O2 sensor off and sprayed the heck out of the CAT to no avail. Still getting one code.
The only code is the P0420 for bank 1 CAT.
A new CAT is on its way to me.
Question: what's the best way to get the bank 1 CAT on the firewall side out? Not a whole lot of room or access from the bottom. I'm assuming from the top is easier after removing the intake manifold? Shows 8 bolts and nuts holding the CAT on, how do I get to the bolts without hurting myself?
Thank you in advance!

First make sure the engine is cold and spray WD40 on the bolts/nuts. Have jackstands under the car. Here is a generic guide that can help walk you through the steps with an image below.

https://www.2carpros.com/articles/catalytic-converter-replacement

Check out the images (below). Please let us know what happens.

Hi Ken,

Thanks for the info, however the CAT I'm replacing is the one that mounts directly on the engine manifold on the firewall side and comes down vertical. Very little space to get to. But I managed! Removed the intake manifold and got to unbolt it from the exhaust manifold from the top. I pulled the entire CAT upward and out.
So, I'm good on the CAT. But now I discovered engine oil all over the Serpentine belt area and also on the opposite side over the Trans case. Looks like the oil pressure sensor and the VVT switch are both leaking oil out! Terrible design and plastic material, wow! The vet was quite easy to get out after removing the fuel rails, but the oil pressure sensor below the oil filter housing is a real pain without a deep 23 or 24mm socket. Wish me luck!

Good luck! If you have issues with this please post your new question here, you must be logged in.

https://www.2carpros.com/questions/new

Cheers, Ken

Hi Ken,

Happy new year!

So, after replacing bank 1 CAT, this is the one next to the firewall, the check engine and p0420 just went away! Car passed emissions test. no codes for couple months.
Unfortunately, last week, the check engine began flashing with p0300 random misfire. diagnosed it to be coils #2 and #4 so i decided to replace all 3 old coils on the Bank 2 side, it was easy getting to them without taking off the intake manifold. After clearing p0300, it came back several times. i pulled over, turned the car off, turned it back on again and was able to drive without any issues. This repeated a few times. right now, i have a solid p0050, p0051 and p0153 codes indicating the o2 Bank 2 Sensor 1, but that one is new. i replaced all 4 o2 sensors, and all 6 coil packs, plus new NGK plugs on the front side. (i've yet to replace the spark plugs on the rear side, pain in the ass to get to).
Is it possible that the Bank2 Sensor1 gone bad, or front CAT now needs to be replaced? car has 167k, no leaks, new air filter, oil change, clean MAF, new 6 coils, new 3/6 plugs, new 4 o2 sensors, one new CAT Bank1 and one OLD CAT Bank2.
i am planning today to plug in my old/used o2 sensor from Bank1 Sensor 1 in place of Bank 2 sensor 1 (those are interchangable by design) in hopes of clearing the p0051 code.
Any other advice is greatly appreciated!

This is a new problem so please start a new question. Please post your new question here, you must be logged in.

https://www.2carpros.com/questions/new

I am glad we got the p0420 code fixed. Thanks for letting us know.

Cheers, Ken