Car Threw A P0420 Code

**iowagrandprix** · 11-17-2014

Hello All,

I'm new to this forum, obviously. So I have a 2005 base model grand prix. Purchased it from my brother-in-law in May 2014. Even before I got it, he had mentioned something about how it kind of sputters when accelerating. Also, sometimes when you go to turn it on, you have to try a couple times because it'll start and then sort of sputter off. I don't know why he never looked into it, but I am more of the investigative type when it comes to cars. I thought maybe it could be an issue with the spark plugs, but I'm no professional. I still have yet to go get it looked at. Shame on me, I know. Anyway, this last Friday my car's check engine light came on, so I scanned the code and got P0420. I'm sure all of you know that it came back saying, "Catalyst System Efficiency Below Threshold (Bank 1). From what I understand, this could be a few different things? Could this be linked to the 'stuttering' when accelerating? What steps should I take in figuring out where the problem is? Any help or suggestions would be greatly appreciated! Thanks!

**Scottydoggs** · 11-17-2014

is there a cat still on the car? how many miles? has it ever had a tune up you know of? if its got over 100,000 miles change the plugs and wires.

**DarkPassNger** · 11-17-2014

My car recently got the same code(although runs fine.. at least for now)...I looked into it some and found some possible reason could be a leak at the manifold...an o2 sensor...plugs and wires as was mentioned and even the cat itself. That's only to name a few ...I did a search and it can be several things(as usual). If you get sorted please reply as to what it was...If I resolve mine , i'll do the same...Good luck.

**primem** · 11-17-2014

hammer fist the cats to see if anything is rattling in there. I've seen broken honeycomb cat element move around on acceleration and cause an exhaust restriction. the fuel trim went ape **** when this was occurring.

**jersanter** · 12-03-2014

I’ve got a 2006 with the same issues. I’ve been having 420 code issues since July. Replaced both O2 sensors and eventually the cat itself. Yesterday, that damn light came on again and my gas mileage is suffering greatly. I can’t figure it out but about ready to park this car in the ghetto and leave the keys in it. If anybody has any suggestions, I’ll listen.

**Scottydoggs** · 12-03-2014

if you installed a high flow after market cat, that could be why. you can try them o2 simulators if its a hi flow cat.

**primem** · 12-04-2014

or a cheap cat with low cerium content...cerium is the oxygen storing element in the cat.

**Slick2500** · 12-04-2014

Originally Posted by Scottydoggs

if you installed a high flow after market cat, that could be why. you can try them o2 simulators if its a hi flow cat.

O2 sims will not work in an 01+ car, because of the cat diagnostic test, the pcm will get stuck in the test mode and peg the long term fuel trims at 16.4.
Read this for more information http://www.grandprixforums.net/threa...ou-should-know

**Impala131** · 12-04-2014

I think Slick and Scotty are talking about two different things.... I think when Scotty is saying, "O2 simulator" he means O2 spoofer. He gives a photo of the good'ol anti-fouler trick (which from what I hear often works). It just keeps the O2 sensor out of the direct exhaust stream so it will report steadier data, as if there was an OEM cat there.

Generally the "02 simulator" term is for an electronic device that pretends to be an O2 sensor and sends the PCM certain data, which does not work on newer vehicles like yours, as Slick is indicating to you.

**Slick2500** · 12-04-2014

Originally Posted by GTP131

I think Slick and Scotty are talking about two different things.... I think when Scotty is saying, "O2 simulator" he means O2 spoofer. He gives a photo of the good'ol anti-fouler trick (which from what I hear often works). It just keeps the O2 sensor out of the direct exhaust stream so it will report steadier data, as if there was an OEM cat there.

Generally the "02 simulator" term is for an electronic device that pretends to be an O2 sensor and sends the PCM certain data, which does not work on newer vehicles like yours, as Slick is indicating to you.

I was kinda confused on that myself, I just wanted to make it clear that the op not run an o2 sim. There are companies that are actually making O2 spacers now.

**4fanintexas** · 12-05-2014

I've got a 2000 GT with the same issue. 295,000 miles and still runs good but has that darn 420 code issue. A couple of months ago it started hesitating under acceleration at highway speeds. For example, while cruising at 60+ it would run fine. But if I gave it enough gas to accelerate but not enough to downshift, the engine would start missing and the SES light would flash (random miss). Then, mysteriously, the issue resolved itself. However, a short time later I started having the p0420 issue. Any thoughts guys?

**Slick2500** · 12-05-2014

Flashing SES light +hesitation while accelerating + p0420 = cat needs replaced.

**4fanintexas** · 12-06-2014

Yep, that's kind of what I was afraid of. Thanks for the reply.

**DarkPassNger** · 12-12-2014

Funny thing my code didn't start until i changed the cat...put on this one in my car...maybe the spacer on the o2 may help me.......also had a **** of a time getting the o2 sensor off the old cat pipe...even had to apply some heat....wonder if it damaged the o2? If i clear the code it doesn't come back sometimes for a couple weeks thou.
https://www.rockauto.com/catalog/moreinfo.php?pk=1316981&cc=1431256

**theaceofspades** · 12-12-2014

Probably higher flowing than gms original cause gm is an ass hat about emissions

**HighOctaneRacing** · 12-12-2014

Originally Posted by Slick2500

O2 sims will not work in an 01+ car, because of the cat diagnostic test, the pcm will get stuck in the test mode and peg the long term fuel trims at 16.4.
Read this for more information http://www.grandprixforums.net/threa...ou-should-know

That's complete bull****. The wife put the ZZP o2 sim in her 03 GP and it worked flawlessly. I put one in a 2001 and that had no problems whatsoever. She no longer has the 03 but will use her sim on her 02 if she doesn't get a tune with the headers right away.

**Slick2500** · 12-12-2014

Did you ever scan it?

Sent from my Galaxy Far Far Away

**Impala131** · 12-12-2014

Originally Posted by HighOctaneRacing

That's complete bull****. The wife put the ZZP o2 sim in her 03 GP and it worked flawlessly. I put one in a 2001 and that had no problems whatsoever. She no longer has the 03 but will use her sim on her 02 if she doesn't get a tune with the headers right away.

I thought this issue happened 100% of the time in 04+ W-Bodies, but only certain years/models between 2001-2003.

**HighOctaneRacing** · 12-12-2014

Originally Posted by GTP131

I thought this issue happened 100% of the time in 04+ W-Bodies, but only certain years/models between 2001-2003.

It IS an issue with 04+ and several members have said they had no issues with the 01-03 years.

IK the 03 was scanned with a snap on scanner and IIRC it was 14.7.

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