Which test(s) did the 172 come from (see my "notes on pulling EEC-IV codes in the EEC-IV fourm for a description of the three types of codes). The 412 and 538 can only come from the KOER test.
KOER 412 is often more symptomatic than diagnostic. It simply means that the engine wasn't running smoothly enough during the KOER test. I've actually gotten this code on one occasion by turning the steering wheel during the KOER test. The additional and variable load the power steering pump put on the engine was enough to trigger this code. Where you have other codes to work with, I'd ignore this one.
KOER 538 almost always indicates operator error during the KOER test (specifically, you neglected to perform the "goose" test). At the appropriate time, the computer expects you to "goose" the throttle, and this code indicates that the computer didn't see it. See my notes on pulling EEC-IV codes thread for more details.
172 can be either KOER or CM or both. Where the CEL has been coming on, I would assume that it is at least showing up in the CM codes, but I also don't like to assume these things. A KOER 172 can sometimes be a result of operator error. The O2 sensor needs to be preheated immediately before the test. If this wasn't done and you got a KOER 172, I'd repeat the test (making sure to preheat the O2 sensor) and make sure it's a real code.
"By the book," the first step in diagnosing O2 sensor codes is to test fuel pressure (see the fuel pressure test mini-diary in the Under the Hood forum). The other place to start is to check the wiring between the sensor and the PCM. The PCM can't usually tell a difference between a faulty sensor and a bad connection.
