I have a 2006 Chevy Cobalt.
The car idles very rough and the engine light is on. I have not gotten the computer scanned for
codes yet. When i accelerate, my car runs great, just idle is bad and hesitates from a stop sign.
Do you know what could be wrong with my car?
Well it sure sounds like you have a misfire condition. Usually this is due to a faulty ignition
component such as a spark plug, or even the coil/ignition module assembly. That would normally
store a code P0300 in the computer.
Since you seem to only be affected at idle, this could actually be an internal engine problem. The
cylinder heads on the 4-cylinder engine have been known to have sticking valves due to carbon
deposits. If that is the case, you would need to have some major engine work done. The head
would need to be removed and either replaced, or at least have the valves that are causing the
problem replaced, if not all of them.
VISITOR: OOO. That sounds expensive. Is that something that would still be under the warranty?
Answer; Well, since the car is 6 years old, probably not under the factory warranty anymore. If you
have an aftermarket warranty, that will surely help. If that is the problem, you can try to call GM for
assistance and sometimes they will help you out if you've been a faithful customer at the
VISITOR: I have been to my local dealership several times in the last few years. Some services that i
got in the mail, some other things when it was under warranty. Do you think they might help? Or do i
need to be a customer that has spent a lot of money with them?
It can't hurt to ask, that's for sure. But first things first, I would take it to them and let them diagnose
it properly. It could be something as simple as a bad spark plug causing the P0300 misfire and not
cost much at all. But if it is more severe, then would be the time to ask for assistance.
VISITOR: OK. If it turns out that the problem is the valve and head thing, can you give me an
estimate of what the P0300 repair cost would be, just so i have a rough idea.
Worst case scenario, if the head itself needs to be replaced, you're going to be looking at least
$1500-$2000 in repair costs. If the valves can be cleaned/replaced only, then you're still going to
be upwards of $1000.
VISITOR: Wow. Thank you so much for your time. This is a great thing you do online here.
You're welcome, and thank you as well! We're here to help!
2006 Chevy Cobalt Rough Idle- Misfire Code P0300
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Below left is a picture of the cylinder head removed. A test is to fill each chamber with fluid,
and then look for the fluid to leak past the valves- see closeup picture on the right.