I have a 06 Armada with 103000 miles. About 3 weeks ago my check engine light came on and I took it to Auto Zone and they told me the code was P0430. Right around the time my check engine light came on my engine felt like it would misfire. My buddy cleared the code for me and it hasn't come back since. My engine also seems to be accelerating fine. Problem now is that I am getting horrible gas mileage, around 10MPG. Most of my driving is highway. I tried to set up an appointment with the dealership and the service advisor told me there was nothing they could do to trouble shoot unless the check engine light is on. Does anyone have any suggestions on what the issue might be or somewhere I can take it to find out what the problem is? I realize that it could be a bad cat, O2 sensor or a cracked manifold but I don't know where to start.
'06 LE Majestic Blue