00 focus se check engine light

my check engine light is on steady but there is no dtc code showing (flashes "none"). any suggestions? i suspect an O2 sensor, but I would have thought this would have thrown a code. any help is appreciated!

i may have cured(?) my problem. after checking the pcv valve, the next time i started the car the check engine light was out. this may have been coincidental or not.

Go to some place like Autozone and have the codes read with a scanner to see if there are any stored codes.