P0420, Please help!!! - Chevy Malibu Forum: Chevrolet Malibu Forums

07Malibu · 07-30-2010, 03:08 PM

My girlfriend has an 07 Malibu LT with the 2.4L engine. A couple months back she had some tranny issues. I think they replaced the 2-4 solenoid, but can't remember for sure. She said she was having tranny problems again, so I took it back to the shop for her and the said the codes were p0300 (engine misfire detected) and p0420. He took it for a test drive and left his code reader hooked up to show current misfires and it wouldn't misfire again. They told me to change the plugs(which I did) and cleared the codes out. The CEL came back on 2-3 days later, and when I took it to Autozone today it showed the p0420 code only, but had it twice. I tested the vacuum in the intake manifold and it tested at 20 in HG at idle. When I rev'd the engine to 2500 rpms slowly, it pretty much held steady, when I rev'd quickly the vacuum dropped to around 7 in HG, but quickly went up to around 25 in HG, and back to 20. The needle was very "wobbly" the whole time, but didn't "wobble" more than 1 in HG either way if that.
So, it seems like the cat is good. But I don't really understand when it says if the needle is erratic it could be worn valve guides, how much it means the needle is being erratic.
Can someone please help tell me what could be wrong?

Oh yeah, the car is very hesitant when trying to accelerate, I know it's a 4 cylinder, and I'm not that familiar with her car, but it seems VERY sluggish. Also, when I changed the spark plugs, I didn't have a gap tool, or the specs for how big or small to gap the plugs. So maybe that could be the problem? Thanks in advance for all of your help!!!

**chevyguy8893** · 07-30-2010, 03:45 PM

P0420 is a catalyst efficiency code. 100% of the time you need a new catalytic converter. If you want to be sure it is the cat pull the O2 sensor before the cat and drive it, the sluggishness will go away because there is no more restriction. Depending on you mileage you may be covered under the 8 year 80,000 mile warranty on it. Also, the cat can cause a general misfire code since it affects the whole engine instead of one cylinder, that's why it is not P0301, P0302, etc.

07Malibu · 07-30-2010, 04:00 PM

When the tranny shop guy hooked up his scanner it said all four cylinders had anywhere between 15-30 misfires. Also, her car has 117k miles. So were well past the emissions warranty unfortunately. My understanding was that the vacuum gauge would show a big drop with very slow rebound during acceleration. But when I tested it rebounded very quickly. Feel free to correct me if I'm wrong, because I'm far from a mechanic. Thanks again for your help. I'll try to pull the 02 sensor and test drive asap, but it's storming out right now. Thanks again!!!

**chevyguy8893** · 07-30-2010, 04:06 PM

I have never used a vacuum gauge to check a cat, so I can't tell you much there. I use the readings off of the Tech II scan tool at work to see how well the cat is performing. I have had a lot of cars come into work with the same code and it has always been the cat.

07Malibu · 07-30-2010, 06:58 PM

Where would I get a replacement cat from? Should I stick with stock and get it from the dealer, or order after market from a parts store or online store? And of course I don't have the right size open end wrench to undo the O2 sensor, and kids are sleeping so can't run to get a bigger one. Guess we'll try to test that part tomorrow.

**DrivenDaily** · 07-31-2010, 06:52 AM

O2 sensor socket is a special one. You might be able to borrow it for free (leave deposit, use, return) from AutoZone or other auto parts store. I have. You may have to buy a part, though.

Most muffler shops can get you a cat. Check prices and warranties. Don't get one from a junk yard. Most states (if not all) it's illegal to buy cats from a junkyard anyhow. And you don't know their condition.

07Malibu · 07-31-2010, 10:27 AM

I got an O2 sensor socket from local Napa, and disconnected the O2 sensor and took it for a short spin. It still seemed very sluggish trying to accelerate. Do you still think it's the cat?

**chevyguy8893** · 07-31-2010, 03:03 PM

I still say there is an issue with the cat itself, but you may have something else wrong. I talked to someone at work today, the vacuum test is not a reliable test on newer vehicles. Best I can say if you want a for sure answer take it to a good deal and pay to have them look at it. Then you can fix it after that or have them fix it.

07Malibu · 08-06-2010, 06:38 PM

I just got a pc based code reader and it reads the O2 sensor voltage among other many other things. The front o2 sensor reading varies as low at .035 V up to .905 V, but had a pretty steady looking up and down wave. But the rear o2 sensor held very steady, starting out at .77 V and bouncing around .005 V either way, then increased as the engine warmed to .86 V and again varying no more than .005 V either way, holding very steady. Does this sound normal or does this help show a problem?

Also, does anyone else have suggestions on where to get a new cat from?

Thanks again for all your help!!!

**chevyguy8893** · 08-06-2010, 07:45 PM

Your oxygen sensors are performing normal, so no issues there. It is normal for the post cat O2 sensor to read steady and the pre-cat to fluctuate. As for an aftermarket cat, it will not work in this situation. On the 2.2L L61 4 cylinder the catalytic converter is part of the exhaust manifold and has to be bought as on piece.

I forgot to add earlier, engine misfire over a period of time destroys the cat. I don't know if you had the misfires before the p0420, but that may have been the cause, or just time and normal wear and tear.