Last summer I had a CEL (01 Prism) and I replaced the gas cap.  It took
several days before the CEL went out.

                 ---MIKE---

A loose gas cap is the most common cause of check engine lights coming on
but the trouble code has to do with the fuel tank evaporative emissions
system, which wasn't one of the codes listed.  The system uses 2-trip
detection logic to determine whether to illuminate the check engine light
(actually called a malfunction indicator light, or MIL).  2-trip detection
logic means that the condition must be present or absent for 2 driving
cycles so if the condition is not present any more, the MIL will go off.

The actual trouble codes would have been more helpful than the descriptions
you gave because the standard On Board Diagnostics generation II (OBD II)
system does not have a trouble code for large vacuum leak or engine oil
leakage into exhaust valve guide seal.  The OBD II codes will tell you which
catalytic converter is bad (a 4 cylinder has 1 converter and a 6 cylinder
has 2 converters) and will also tell you which cylinder or cylinder is/are
misfiring and how severely.  There is also no code for a defective
converter - the code will say what the converter is not doing.  A bad O2
sensor can give you a code that says that the catalytic converter operation
is below threshold.

The MIL can go out because someone cleared the trouble codes when they
pulled them or because the condition is intermittent (2 trip detection
logic) or the condition is gone.

For now, don't worry, and if the MIL comes on again, take it to a dealer.
Some emissions components are covered for up to 80,000 miles in the U.S.

The check engine light will illuminate when the process controller can not
keep the fuel/air ratio within it design parameters.  Something as simple as
a fouled plug can be the cause.  If the fault clears itself after a
predetermined number of restarts, the light will go off.   At that mileage a
temporary glitch, like a piece of carbon, was the most likely scenario.  Not
to worry, if it happens again take the vehicle to a dealership for
correction, WBMA

mike hunt