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.

that's the sensor before the catalytic converter. And the radiator caps do come off cold with no harm, if you can't get it off, have a pro like me do it with a giant pair of pliers or cut the old one off carefully and then buy a new one