2006 Chevy Cobalt P0420 - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 7 (permalink) Old 02-17-2014, 06:48 AM Thread Starter
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2006 Chevy Cobalt P0420

Hello everyone I am looking for some assistance with my mom 2006 Chevy Cobalt with a 2.2 L 134 CID L4 DOHC 16V ECOTEC and 157 k miles. It is kicking a p0420 code and sometimes it kicks a p0300. Here is what happened yesterday. She drives like a Grandma so I told her she need to get out on the interstate and run 60-80 for a couple of miles and see if she could blow out the converter so It would pass emissions. She got up to about 75 for a mile or 2 and the check engine light started flashing. I checked the codes and it was a po300. I cleared the codes and took back off and it started getting a p0420. I cleared again and got a p0420 again. I did this 3 more time and each time p0420. I am assuming the converter is shot since it has 157k miles but my biggest question would a bad converter cause a p0300? I am going to get the car this weekend and change the converter and give it a tune up but I want to know what other stuff I should check. I have a code scanner and a Genesis Scanner but never used it. I also put in a can of seafoam in the tank this weekend.

Here is my plan.
Hook up scanner and monitor o2 sensors before changing the Converter and if they are showing ok change the converter.
Change Converter
Change Fuel Filter
Check Fuel Pressure
Change Oil
Change Air Filter
Change Plugs/Wires
Check for Vacuum Leaks
Check Coil? How do I test it?

Can you think of anything else I am missing or anything else I can do? I have to get this thing to pass emissions by Feb 26th.

Any help will be greatly appreciated.
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post #2 of 7 (permalink) Old 02-17-2014, 08:44 AM
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P0300 is an unspecified missfire. I would change the spark plugs first if they have never been changed. Do one job at a time so you know what solved the issue. P0420 could be caused by unburnt gas because of poor plugs.

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post #3 of 7 (permalink) Old 02-17-2014, 08:51 AM
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I was thinking the same thing. Changing the cat will not solve the problem, because something has caused it to fail, likely the misfire. The stock ACDelco iridium plugs are recommended to be replaced after every 100k miles, in which you're way overdue. Hope it's as simple as that. Good luck.

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post #4 of 7 (permalink) Old 02-17-2014, 08:52 AM Thread Starter
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Ok sounds good. I will give it a try and see what happens. I am pretty sure the plugs have never been changed. Thanks.
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post #5 of 7 (permalink) Old 02-17-2014, 08:59 AM
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Good. For best results, stick to the stock ACDelco iridiums. They're the best plugs for our cars and would be worth the extra price over budget brands.

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post #6 of 7 (permalink) Old 02-21-2014, 06:38 AM Thread Starter
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Ok everyone I am here to report back. I checked the car out and I found that the coil pack was actually cracked and the spark plugs were completely wore out. After I replaced both I have yet to get another misfire and running smooth. I then went to check the converter and both o2 sensors were showing the same reading on the scanner so I pretty much knew the converter needed replaced. Last night I replaced the converter, changed oil and fuel and air filter. While I was changing the oil I noticed a oil leak near the starter area and the starter was covered up. I was too tired to really investigate last night but I did some googling this morning and saw that it is possibly the oil pressure switch. Has anyone had this problem and can you tell me exactly where it is located and how hard to change. Other than that do you think my diagnosis on the misfire and coil are correct. I am going to attach the coil pic. below.

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post #7 of 7 (permalink) Old 02-21-2014, 07:17 AM
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I'd say you found one problem. Oil pressure sensor is right above the starter.

---------- Post added at 11:17 AM ---------- Previous post was at 10:55 AM ----------

Engine Oil Pressure Sensor and/or Switch Replacement
Removal Procedure
Disconnect the negative battery cable. Refer to Battery Negative Cable Disconnection and Connection .
Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle .
Disconnect the engine oil pressure sensor electrical connector.



Remove the engine oil pressure sensor.
Installation Procedure
Notice: Refer to Fastener Notice in the Preface section.





Install the engine oil pressure sensor.
Tighten
Tighten the oil pressure switch to 22 Nm (16 lb ft).

Connect the engine oil pressure sensor electrical connector.
Lower the vehicle.
Connect the negative battery cable. Refer to Battery Negative Cable Disconnection and Connection .

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