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Discussion Starter #1
My 06 Cobalt (2.2 DOHC) was throwing a p0300 code for about two months before I got it fixed. It ran fine for about two weeks and is now throwing a p0420.

I cleaned the throttle body, no luck. It also has a high idle, around 1400.

Did the CAT go bad running it with the p0300 code?

Is there a way to test the CAT?

What is the best replacement CAT to get and where?

Thanks.

Joel

---------- Post added at 05:56 AM ---------- Previous post was at 05:44 AM ----------

Was wondering if it could be the "Mass air flow sensor". I did spray some electronic part cleaner on it, is there a way to test this?
 

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The p0420 code is catalyst efficiency is bad. From the mechanical/electrical misfiring of unburned fuel particles will go straight to your catalytic converter. Hydrocarbons aka Fuel atomization from the injectors clogged up your cat converter and causing the rear oxygen sensor to not read the right amount of c02 levels will trigger that code and run into "limp" mode. Its nothing more than just staying in open loop status to run rich to keep your vehicle alive.

If you were planning on going abouts a new cat converter and Bankerbalt's idea of a magnaflow performance cat, it "must" be a 3-way platinum bedding style of cat in which they offer. I would highly suggest fixing your p0300 (random engine misfire) first, otherwise that new cat will go bad once again. The p0300 can be from improper ignition timing, clogged injectors, wrong octane level, something that is not meeting the original setup for the combustion cycle of the engine.

I know GM offers a full downpipe with the cat converter built in, but its asking around $840 new. Or just pull one out of a junkyard for a lot less than that, but you risk on if the cat is good or not. ZZP offers a 2.5" catted downpipe and people on here love it for the non p0420 code, just include the 400 cell cat they offer under the options if you plan on purchasing one. Hope this helps.
 

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Thanks for the replies.

The p0300 code is gone, replaced plugs, boots, module.

Would a bad CAT cause it to have a high idle?
 

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Thanks for the replies.

The p0300 code is gone, replaced plugs, boots, module.

Would a bad CAT cause it to have a high idle?
Glad you got that code gone, Your car is in open loop mode with a CEL on. For your p0420 Catalyst Efficiency code, your car may act a bit funny, such as engine temp will not set to normal operating temps, go through gas much quicker and fuel economy decrease up to 5 mpg's, fuel odor coming by tail pipe, and possible idles of 900 rpms-1100.

If your idle is sitting after 5 minutes above 1200, you have something else that's causing it, vacuum leak, dirty air filter. Just check your hoses and air filter to see if any dirt/debris or cracks, abnormal wear.
 

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no problem. Typically a vacuum leak will not trigger a DTC or CEL since its mechanical.
 

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The idle has settled down, around 900. Took everything off after the throttle body and made sure there were no leaks. Also changed the air filter.

However the CEL for code P420 is still on.

A new sound arose, at around 35 MPH (and up) the car makes a hum, loud enough to be heard over the radio. The hum is most noticeable when the car is cold.
 

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p0420 will not be resolved until a new cat converter is installed and code has been cleared. Good to hear your idle is now roughly in the right area, is any accessory turned on such as A/C or heat? That will cause an engine load to increase RPM.

My car does that too, but at 5th gear on 45 mph area, but mine shakes the roofline, but that's caused by harmonics from the transmission. You have to realize that your car has lots of vibrations and hums that the human ear cannot hear nor feel. Sometimes it can be from gears or mechanical moving parts, like for a good example, a camshaft spinning without pistons compressing is annoying loud, we hear it on a engine dynamometer when breaking in a camshaft for racing applications, but when the engine is completely turned over and running, you wont hear the camshaft anymore.
 
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