Get a scanner or go to some place where they do it for free.
See what code it is.
Look on any Helms / Chilton's Manual for a feedback on the code.
Dont just go replacing parts, look at what they say about the
code and test everything.
"Low Voltage at o2 sensor" doesnt mean "Replace the o2 Sensor"
Check to see why is getting low voltage.
Capish!

Also, if the Check Engine light is just lit it means you have a
problem with the Emissions. If the light keeps flashing
then it means you have engine misfiring, which is bad.

So just go about doing that...