CEL after maintenance

[bglz42]

Bought a very clean, one-owner and well-maintained ’01 Boxster S with 38k miles. Ran great!

Changed the air filter after about 500 miles. Popped the first CEL . Code read aging O2 sensor bank 2. Cleared, ran fine. Would display CEL on occasion and, while I waited on the parts to arrive, I would just clear them out with my OBD II tool.

Changed the sensors on Sunday. Started, and immediately began running rough and blinking CEL came on. Read code, P0302. Bad coil possibly? After changing the O2 sensors?

Thinking of possible causes: all the way back to replacing the air filter. Could I have installed it incorrectly, maybe rolled the gasket and air is getting in? It looks fine, but I might buy another and try again. After I change that coil out, of course…

Might order a Durametric and dig a little deeper...

Another data point: the left side upstream O2 sensor tip was completely white in color. The right tip was dark reddish... don't know if that means anything but it stood out to me...

Any ideas?