P0420, Please help!!!
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!!!