I'd guess that when the CEL goes on, it's running in open loop mode because it thinks the O2 signal is invalid.
Open loop is usually richer.
When the CEL turns off, it thinks the O2 sensor signal is once again valid, and runs in closed loop. (leaner).
Thank you Ed for answering my questions. I guess I can play with the truck while I am driving it to make the CEL go off. Just kind of annoying.
