08 Denali... P0420 Error Code.

[probablecause]

Looked up the error code and it says that my #1 cat has issues. Went to Dr. Google and said to replace O2 sensor. However, most say the CAT is shot. With that being said:

Replace the #1 cat only OR replace both with aftermarket cat system. My boat mentality is to throw on headers and a cat back muffler system also. Any recommendations for a complete system or put-together system?

 

[wsuwrhr]

Are you in CA?

Looked up the error code and it says that my #1 cat has issues. Went to Dr. Google and said to replace O2 sensor. However, most say the CAT is shot. With that being said:

Replace the #1 cat only OR replace both with aftermarket cat system. My boat mentality is to throw on headers and a cat back muffler system also. Any recommendations for a complete system or put-together system?

 

[probablecause]

yes, Brea.

 

[rush1]

Better buy the correct cat or you won't pass smog at a lot of shops

 

[wsuwrhr]

Yep, that is why I was asking. You have to buy a 50 state legal cat or cats, otherwise it will be further headaches from here on out.

How many miles are on it, cats should still be good on an 08.

I would replace the O2 sensor first, or if you don't work on your stuff, take it to a dealer.

Brian

yes, Brea.

Better buy the correct cat or you won't pass smog at a lot of shops

 

[77charger]

O2 sensor only takes a few minutes to replace not a big deal.I have had to replace 2 on my 2500hd truck has 184k.

 

[ElAzul]

CARB just said cha ching

 

[wsuwrhr]

CARB just said cha ching

As far as I know, Emissions is mostly Fed now anyway, it isn't like it used to be. Cars roll off the same, no matter what state they are headed.

Being in another state, or some counties, means you don't have to put 50 state parts back on, but the letter of the law says they should still be on.

Brian

 

[DRYHEAT]

Could it be under warranty? I thought anything emissions related had a 10 year warranty?

 

[LargeOrangeFont]

If you are going to AZ or NV soon pick up the 49 state cats for less than half price.

There is no sniffer test for that car, just visual and OBD.

 

[Rsqfxr]

If you are going to AZ or NV soon pick up the 49 state cats for less than half price.

There is no sniffer test for that car, just visual and OBD.

my last smog on my commuter they noticed the aftermarket cat put my car on a lift and ran the part number, lucky it was legit, i now cross reference all the vehicles i repair that need cats on the carb website

 

[wsuwrhr]

That works, right up up til it won't pass a visual. Ask me how I know this.

If you are going to AZ or NV soon pick up the 49 state cats for less than half price.

There is no sniffer test for that car, just visual and OBD.

 

[LargeOrangeFont]

my last smog on my commuter they noticed the aftermarket cat put my car on a lift and ran the part number, lucky it was legit, i now cross reference all the vehicles i repair that need cats on the carb website

Good god. Time to find a lazier smog guy

On my old DD the last smog I did for it I took it to a new place and he finally noticed it had a header. It passed 3 other Smog’s that way.

 

[Runs2rch]

Change upstream and downstream for the bank. If it comes back go to bigdaddiesgarage.com

They make a simulator and a bung that just pulls the rear O2 just out of the stream. The loading of the cat could be weakened, but 99.99 percent it is fine.

 

[HALLETT BOY]

Good god. Time to find a lazier smog guy

On my old DD the last smog I did for it I took it to a new place and he finally noticed it had a header. It passed 3 other Smog’s that way.

You have no idea how hard the BAR likes to nail “lazy” techs , the fines are astronomical !

 

[LargeOrangeFont]

You have no idea how hard the BAR likes to nail “lazy” techs , the fines are astronomical !

I do know the fines are steep, but I rarely encounter one that fully does their diligence. Of course when I do it is at the worst possible time.

 

[Bigbore500r]

I'd swap the 2 post cat sensors side to side, then clear the code. If the MIL comes back and it now says "bank 2 cat efficiency below threshold" you know it's the sensor - the problem moved with the sensor. If it comes back as Bank 1 again - you need a cat. As far as buying a new cat to fix the issue - I'd fire up HP tuners and just turn that MIL code to "not reported" and never see it again lol. Pass CA smog all day!

 

[Bigbore500r]

Good god. Time to find a lazier smog guy

On my old DD the last smog I did for it I took it to a new place and he finally noticed it had a header. It passed 3 other Smog’s that way.

I used to work for Kenne Bell in the early 2000's, we had a shop car that was going thru CARB testing for a blower kit. We decided a new cat would help it get thru, but we weren't allowed to throw a new cat on it. We ended up "distressing" a new cat, wiping grease and oil on it, burning it in, a few rock chips....etc till it matched the patina of the OE exhaust it was tying into. Aaaaaand Carb certified the kit is now

 

[LargeOrangeFont]

I used to work for Kenne Bell in the early 2000's, we had a shop car that was going thru CARB testing for a blower kit. We decided a new cat would help it get thru, but we weren't allowed to throw a new cat on it. We ended up "distressing" a new cat, wiping grease and oil on it, burning it in, a few rock chips....etc till it matched the patina of the OE exhaust it was tying into. Aaaaaand Carb certified the kit is now

Winner! I've done that on 2 cars I put a new main cat from Nevada on my RX7 when it still had the rotary. Hit it with vinegar and let it sit out in the sprinklers for a couple days. Put a little rubbing alcohol in the tank for the smog test and it was squeaky clean

I put a Magnaflow catted X pipe on my 04 Cobra before these new cat rules came out 8-10 years ago. The car passed the sniffer, but the guy was going to fail me on the visual because the cats were not legal as of that date, but were legal when installed on the car. I remember we got in an argument about it and I finally asked -

"Does the law state that every car with a replacement cat before the law went into effect has to get it removed and must install a CA compliant catalyst to pass smog?"

Him - "Well no that would be ridiculous."

"So if replaced a cat with a another legal cat before the new law went into effect, it is therefore legal right? Because it only applies to catalysts installed after the new law went into effect, right?

Him - "Yes."

"What are we talking about again?"

PASS.

 

[LargeOrangeFont]

I'd swap the 2 post cat sensors side to side, then clear the code. If the MIL comes back and it now says "bank 2 cat efficiency below threshold" you know it's the sensor - the problem moved with the sensor. If it comes back as Bank 1 again - you need a cat. As far as buying a new cat to fix the issue - I'd fire up HP tuners and just turn that MIL code to "not reported" and never see it again lol. Pass CA smog all day!

This. I'm sorta doubting the cat went bad.

 

[SBMech]

I'd swap the 2 post cat sensors side to side, then clear the code. If the MIL comes back and it now says "bank 2 cat efficiency below threshold" you know it's the sensor - the problem moved with the sensor. If it comes back as Bank 1 again - you need a cat. As far as buying a new cat to fix the issue - I'd fire up HP tuners and just turn that MIL code to "not reported" and never see it again lol. Pass CA smog all day!

This is the way to go about it if you do not have an infrared thermometer. If you do, easy way to test is that the output flange on the cat has to be at LEAST 100deg hotter than the intake if the cat is working, I like to see 150+.

 

[dribble]

Swapping to two downstream sensors is the best advice. If it shows back up on the same side you know it's the cat. Approved aftermarket cats are available for that vehicle fairly cheap. Another trick is to create a big vacuum leak (ie PCV or power brake hose) rev it real quick then put the hose back. This will then flood the cat with fuel and cause it to heat up possibly burning any coating on the substsate. Putting headers on an 08 Denali would be a waste IMO.

 

[Classic Daycruiser]

O2 sensor usually fixes the problem.

 

[probablecause]

OK, I will start with the O2 sensor first. Just a question. Bank 1 is the driver's side cat? You are saying swap the 2 "downstream" sensors. What does that mean?

 

[HALLETT BOY]

Why don’t you hook up a scanner and monitor data and then decide if cat is bad or not ? The data will tell you .

 

[LargeOrangeFont]

OK, I will start with the O2 sensor first. Just a question. Bank 1 is the driver's side cat? You are saying swap the 2 "downstream" sensors. What does that mean?

Every newer car has 2 O2 sensors. one in the manifold, and one after the primary cat, before the main cat. The one between the 2 cats is the "downstream" sensor. It has nothing to do with the fueling of the car. Its only job is to tell the PCM that the cats are functioning. If the 02 is bad the car wont run any different. If the cat is bad, and the o2 is good, then the car could suffer driveability issues.

In your case since it is a v8 you will have 2 upstream and 2 downstream o2s, a set for each bank of cylinders.


The upstream 02s handle closed loop fueling. If those go bad, driveability and fuel mileage will suffer.

 

[probablecause]

Ok, so downstream is to the rear of the cats. Got it. If one is bad, it should change from a P0420 to ?

 

[Bigbore500r]

OK, I will start with the O2 sensor first. Just a question. Bank 1 is the driver's side cat? You are saying swap the 2 "downstream" sensors. What does that mean?

Bank 1 is drivers side, and the sensor in question is the one "after" the catalytic converter. There only function is to check converter efficiency, hence when they go bad (or when the converter goes bad) this code will be set. Pull out the 2 sensors located after the cats and swap them sides, see what happens. 100% you will find out if you need a cat, or just a sensor.

 

[Bigbore500r]

Ok, so downstream is to the rear of the cats. Got it. If one is bad, it should change from a P0420 to ?
View attachment 626195

I believe the code for Bank 2 / passenger side is PO430. So if you swap sensors , clear the P0420 code and it comes back in a few days as P0430, you need a sensor. If it comes back as P0420 - you either need a cat, or a hack saw and to move out of CA

 

[Cole Trickle]

Lots and lots of on blaster and go easy removing .... o2 threads like to go away when you are removing them

 

[Maestro]

Sensors are working. Cats failed from something, rich/lean or contamination. Find what failed the cat and fix it. A new cat will pass but if something is contaminating it will fail again..

 

[rush1]

You can't even smog it until the monitors clear so if it's due for smog you better fix it and do the gm drive set for monitors .

 

[probablecause]

Lots and lots of on blaster and go easy removing .... o2 threads like to go away when you are removing them

Should I use some copper anti seize when putting them back on?

 

[probablecause]

do the gm drive set for monitors

???

 

[707dog]

I believe the code for Bank 2 / passenger side is PO430. So if you swap sensors , clear the P0420 code and it comes back in a few days as P0430, you need a sensor. If it comes back as P0420 - you either need a cat, or a hack saw and to move out of CA

yes p0430 is passanger bank 2 i have that code on my 08 esv just haven't climbed my azz under there to swap around sensors came on 3 weeks after smog in dec so no rush beside the dummy dash light

 

[Cole Trickle]

Should I use some copper anti seize when putting them back on?

Anti seize would be a good idea.

 

[probablecause]

I don't need to disconnect the negative battery cable or anything wazoo like that do I?

 

[Bigbore500r]

I don't need to disconnect the negative battery cable or anything wazoo like that do I?

. Just make sure key is off, you’ll be fine

 

[rush1]

???

If you have a check engine light on the computer turns on a monitor and checks itself so even if you correct the problem and turn the light off you have to drive it for a while so the monitors turn off ,you can't pass smog with the monitors on , gm has a specific drive test that will reset the monitors faster than just driving it . Google gm monitor drive test it it will tell you how to drive it

 

[LargeOrangeFont]

If you have a check engine light on the computer turns on a monitor and checks itself so even if you correct the problem and turn the light off you have to drive it for a while so the monitors turn off ,you can't pass smog with the monitors on , gm has a specific drive test that will reset the monitors faster than just driving it . Google gm monitor drive test it it will tell you how to drive it

Or go on Amazon and buy a OBD scanner for $20 shipped that connects to your phone, or a hand held style unit for $30-$40 bucks. You don’t have to mess with disconnecting your battery, key cycles, or drive test loops ever again.

Here is a basic handheld one that will let you reset codes for under $13 shipped. You’ll just have to look up the code online, it does not give you a description like the more expensive units.

OBD2 Scanner OBDII Code Reader - Scan Tool for Check Engine Light - MS300 Universal Diagnostic for Car, SUV, Truck and Van https://www.amazon.com/dp/B011Q18D14/ref=cm_sw_r_cp_api_f8ROAbKYB10P1

 

[SBMech]

Or go on Amazon and buy a OBD scanner for $20 shipped that connects to your phone, or a hand held style unit for $30-$40 bucks. You don’t have to mess with disconnecting your battery, key cycles, or drive test loops ever again.

Here is a basic handheld one that will let you reset codes for under $13 shipped. You’ll just have to look up the code online, it does not give you a description like the more expensive units.

OBD2 Scanner OBDII Code Reader - Scan Tool for Check Engine Light - MS300 Universal Diagnostic for Car, SUV, Truck and Van https://www.amazon.com/dp/B011Q18D14/ref=cm_sw_r_cp_api_f8ROAbKYB10P1

You still have to set the monitors. There is no way to bypass the driving part of it. Usually a 15 minute drive on the freeway, with 15 minutes of downtown type driving will set them. The evap one will not set unless specific parameters are met, has to be between 1/4 - 3/4 fuel level, usually has to sit till completely cold (overnight) to run the tank pressure test ( has to run twice to pass) etc.

 

[LargeOrangeFont]

You still have to set the monitors. There is no way to bypass the driving part of it. Usually a 15 minute drive on the freeway, with 15 minutes of downtown type driving will set them.

Correct. You can’t just clear a code in the driveway of the smog place.

I was 550 miles from home 2 weeks ago and my brand new car threw a CEL. I figured it was a gas cap CEL (it was, I checked and cleared it when I got home). I cycled the “key” probably 20 times, removed and replaced the cap, topped off with gas, and after 8 hours of driving it still did not go off. I cleared it when I got home and it’s has been fine since.

 

[wsuwrhr]

Yea? Did I miss what you bought?

Please tell me you still have the Caddy.

Correct. You can’t just clear a code in the driveway of the smog place.

I was 550 miles from home 2 weeks ago and my brand new car threw a CEL. I figured it was a gas cap CEL (it was, I checked and cleared it when I got home). I cycled the “key” probably 20 times, removed and replaced the cap, topped off with gas, and after 8 hours of driving it still did not go off. I cleared it when I got home and it’s has been fine since.

 

[LargeOrangeFont]

Still have Caddy,

Bought a new DD with a 6 speed manual.

Yea? Did I miss what you bought?

Please tell me you still have the Caddy.

 

[wsuwrhr]

......

View attachment 626323

Still have Caddy,

Bought a new DD with a 6 speed manual.

 

[LargeOrangeFont]

......

It is already almost 1/4 paid for via funds not my own.

 

[wsuwrhr]

@pronstar has a similar story.

It is already almost 1/4 paid for via funds not my own.

 

[rush1]

Or go on Amazon and buy a OBD scanner for $20 shipped that connects to your phone, or a hand held style unit for $30-$40 bucks. You don’t have to mess with disconnecting your battery, key cycles, or drive test loops ever again.

Here is a basic handheld one that will let you reset codes for under $13 shipped. You’ll just have to look up the code online, it does not give you a description like the more expensive units.

OBD2 Scanner OBDII Code Reader - Scan Tool for Check Engine Light - MS300 Universal Diagnostic for Car, SUV, Truck and Van https://www.amazon.com/dp/B011Q18D14/ref=cm_sw_r_cp_api_f8ROAbKYB10P1

They don't make a tool to turn off Obd2 monitors only clear codes that's 2 different things

 

[Bigbore500r]

They don't make a tool to turn off Obd2 monitors only clear codes that's 2 different things

Except HP Tuners, EFI Live...etc - I can make those monitors say whatever we want. 02 monitors ready? Sure!

 

[LargeOrangeFont]

Except HP Tuners, EFI Live...etc - I can make those monitors say whatever we want. 02 monitors ready? Sure!

Correct. I have no O2s at all and no MAF in one car.. it passes all readiness tests and no CEL