[BTRobertson]

That code usually appears on older cars when the cat is either bad or broken. So, given this info, the O2 sensor may be too far away to sense an exhaust stream and is throwing the code because it thinks the cat is malfunctioning. It's not the same as the other code which comes on when the exhaust fumes are too high - this code usually means the cat isn't there because the exhaust gases are too high for a cat to be present. My guess is that your O2 sensor is too far away from the exhaust stream in your defouler, but that's my best guess.

The P0138 code is a voltage short in the O2 sensor. Usual causes are a wiring or O2 sensor malfunction.

The P0139 code is a "slow response" in the Secondary O2 Sensor - this is the one that most people with race headers and a defouler get if the defouler isn't properly spaced from the exhaust flow.

All of these codes are probably expected or assumed to be caused by a race header and defouler, I'm just curious to see if someone with a Vibrant RH w/HFC and defouler get these codes.