2004 Grand Prix GT - P0420 - Video of Symptoms

**04PrixxGT** · 11-12-2012

Hey guys i know there's a thread already posted about this exact code , but the user in question has an aftermarket exhaust setup from what i understand , and that thread veers off to many different solutions according to his exhaust components. I didn't want to hi jack that users thread so i wanted to make a quick post about my vehicle. It's an 04 Prix GT , has over 300,000 km's on it and is 100% bone stock. The car started out having a miss fire , which ended up being bad wires.. I replaced the wires , plugs and coils cause i got a good deal on the coils and figured why not. The car ran great for awhile then it started throwing P0420. I included a video of the issue in question which explains in detail whats going on.

2004 Grand Prix PO420 - YouTube

I put a fair chunk of money into this car over the summer this year trying to freshen it up , spending more time outside with the prix then my friends and family lol

Replaced this year -

HVAC Fan -
NGK Plug wires -
E3 Plugs -
Thermostat -
Coolant temp sensor -
Sway bar bushings -
Sway bar links -
Control Arm bushings -
Leaky rear brake caliper -
Tran filter , trans pan gasket ( leaky ) -
Coil packs -
Drivers Side Axle ( Causing shake ) -
Drivers Side Hub -

Needs Currently -

- Oil pan , only thing on the car rotted and leaky ( wtf ). Patched with JB weld for now until i have the courage to tackle it.
- Rear Strut
- Mufflers both have small holes where the brackets are connected , will be swapping them out. ( And probably the cat as well ) when i get the cash.

I should probably sell the car and get myself an 04+ GTP with lower mileage like i want , which i might do.. But if i can get one more year outta the car i wouldn't mind it. Regardless the car has high mileage and these things are to be expected.. let me know what you guys think lol.

**Burbman** · 11-12-2012

Sounds like your cat is partially plugged.

**Fivefingerdeathpunch** · 11-12-2012

Those E3 plugs are garbage. I would replace with stock delco plugs.

But more often than not when the car throws the P0420 code the cat converter no longer good.

**04PrixxGT** · 11-12-2012

Originally Posted by Fivefingerdeathpunch

Those E3 plugs are garbage. I would replace with stock delco plugs.

But more often than not when the car throws the P0420 code the cat converter no longer good.

You're not the first person to say them plugs are junk , now they're always at the back of my mind to once again replace. lol

**Casper00693** · 11-12-2012

Def. sounds like a clogged cat. I just had that code myself and ended up cleaning my cat out and the code went away but I dont recommend to clean it the way that I did which was an entire can of seafoam down the downpipe. =p

**smoke14** · 11-12-2012

I had E3's in my car and developed a misfire, changed them out and solved my problem. Waste of money IMO.

**04PrixxGT** · 11-12-2012

Originally Posted by smoke14

I had E3's in my car and developed a misfire, changed them out and solved my problem. Waste of money IMO.

lol , well you guys got me convinced they're junk.. Gonna swap em out for something else next weekend , i actually changed over to the E3's about 2500 KM's ago , i will swap them out for something else.

**Fivefingerdeathpunch** · 11-12-2012

Please go with the iridiums. Do not buy platinums or anything else. Our ignition system does not like them.

**KeithGTP03** · 11-13-2012

Looks like you replaced everything besides what the code is pointing to.

**REDCRAPGP** · 11-13-2012

My money is on a bad Cat..also get rid of those E3 plugs.

**SlowNA06** · 11-13-2012

300,000 km, eh?
Cats start to go bad at ~ 240,000 km

**04PrixxGT** · 11-13-2012

Originally Posted by SlowNA06

300,000 km, eh?
Cats start to go bad at ~ 240,000 km

Does anyone recommend ZZP's cats as a sufficient replacement?

**Scottydoggs** · 11-13-2012

id ditch them plug first, see how it runs after that, if its still not up to par go the for the cat.

you can also get $2 copper core plugs for your car, only down side is they only last 15,000 miles.

iridium last up to 90 ish, and cost about 6 a plug.

and if your all stock, look for a walker stock replacement cat or something. high flow cats will set off a code most of the time.

**04PrixxGT** · 11-17-2012

Originally Posted by scottydoggs

id ditch them plug first, see how it runs after that, if its still not up to par go the for the cat.

you can also get $2 copper core plugs for your car, only down side is they only last 15,000 miles.

iridium last up to 90 ish, and cost about 6 a plug.

and if your all stock, look for a walker stock replacement cat or something. high flow cats will set off a code most of the time.

I'm gonna pick up a walker replacement sunday , and try that....

Meanwhile , i just found a 2004 Prix GTP , Comp G.. I could pick it up for 2,800$..It's got 201,000 km's 124,000 Miles. It feels solid , motor sounds good.. Older guy owned it said he brought it in to the local dealer to have them replace the belt and oil bout 15k ago.. ( Whatever ) lol , Point is.. Does this sound like a decent deal? Car is 100% Stock , not even tint. Thoughts on this? I'm skeptical otherwise i really would like to pick it up.

Only problems it has is the sunroof is jamming about half way before completely opening , and for whatever reason there is no audio coming from the speakers.

(Sounds Minor )

**SlowNA06** · 11-18-2012

For $2800, I'd say it's with whatever's wrong with the transmission.

**04PrixxGT** · 11-18-2012

Originally Posted by SlowNA06

For $2800, I'd say it's with whatever's wrong with the transmission.

Yeah that's exactly what scares me lol , odd thought there's 3 - 4 comp g's in my area for sale right now , aswell as an 05" GXP with 98k i could grab for 9,800$.. Choices , Choices. I need something newer but i don't wanna commit. haha.

**DirtDog** · 11-18-2012

For the GXP I have one myself and love the thing. Although their known for trannys to go out easy and quickly. I haven't had any problems major outta mine except for cv half shaft leak. They get good gas mileage or at least mine does

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**04PrixxGT** · 11-18-2012

Originally Posted by marines542

For the GXP I have one myself and love the thing. Although their known for trannys to go out easy and quickly. I haven't had any problems major outta mine except for cv half shaft leak. They get good gas mileage or at least mine does

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Not gonna lie i cant get the GXP outta my head , it's the same color as yours in fact.. I been reading the GXP gets slightly better fuel mileage highway then a GTP? Might be crazy , I'm not sure. I love the GXP , but i don't wanna get a fuel monster either. Ive always wanted one.

**DirtDog** · 11-18-2012

On fuel in the city it gets same as my old 04 grand prix does. On highway I usually get average of 33 or so. I love mine. Just check everything out on it and make sure it ain't got no problems

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**Kreen1k** · 11-24-2012

An inefficient fuel burn could overwhelm the cat resulting in a P0420. NGK makes a nice Laser Iridium to go with your wires. What I use on all my vehicles regardless of make. Replace the plugs first before the cat, see what happens.