Car Idling Rough and Turning off Sometimes at Stop Light

[Curiouspanda]

Hi today my car's check engine light came on and started driving kind of rough and idling kind of rough. The car shut off on me twice while idling at a light and had to restart it. My car is a 2007 Chevy Cobalt. I managed to drive to Auto Zone where they said the codes were P03000 & P0420.

I googled the codes together and saw that some people said this may be a catalytic converter issues along with an o2 sensor. However on top of the Auto Zone print out it says that its most likely a fuel pump that needs to be replaced. Any idea based off your experience what this might be?

Trying to get an idea of how much I'm gonna have to spend.

Thanks in Advance for the HELP!

[tomj76]

I'm sorry that your post didn't get any response sooner. I had to replace my calallytic converter too, but I saw different symptoms.


In my case, when I removed the converter I could see down to the first ceramic unit and it was clearly melted. This can happen if the mixture sends a lot of unburnt hydrocarbon into the cat. I believe it happened to me because of a faulty valve timing solenoid.


My symptoms before replacing the cat was that for a long time I had a code indicating a problem with the VVT solenoid. I didn't replace it because the only issue was that the engine didn't idle well at stoplights. Sometimes it would stall but most of the time the engine RPM would repeatedly surge.


Later, I noticed a big drop off of power from the engine. Basically my acceleration was sluggish, but I compensated by staying in lower gears (I have a manual) when accelerating. I mainly noticed it when cruising at highway speed and needing to down shift to make even smaller hills.


To diagnose, I removed the upstream O2 sensor and put a pressure gauge in the hole. It didn't look very high when I ran the published test, maybe less than 1 psi, but if I revved the engine I saw large pressure spikes. Then I removed the cat and looked into the inlet flange and saw the melted ceramic.


I believe my problem came from the unresolved VVT solenoid, so that was also repaired. This occurred several years ago and I have not seen any issues since.