Hi Scotty. I have to say after searching the internet, your site is my favorite! I have a 2004 Pontiac Aztek with almost 223,000. I am unable to unscrew the radiator cap. Other have said NEVER open the cap just add to the reservoir. Can that be right? Also, I have a P0135 Error - I know it's the O2 Sensor (Bank 1 / Sensor 1), which one exactly is that because I'm so confused. Thanks in advance for your help.