Hyundai Repair: CEL and poor gas mileage, poor fuel economy, obd ii code reader

Question
I have an 02 1.5L Accent and recently have been having a plummet in gas mileage(from 34 mpg to about 24 mpg).  I had to replace  the cylinder head and timing belt as the result of a recent Tbelt break, but I have been driving the car with no kinks except for squealing belts at startup.  Two questions: Can mis-tightened belts (I think it is the PS pump belt) cause the CEL, or do you think it has to do with my poor gas mileage and how might I go about fixing it?

Answer
Unless the loose belts caused your system voltage to be low or the vehicle to overheat (not likely), I cannot think of any reason they should cause poor fuel economy or a check engine lamp.

Your first step should be to read the diagnostic trouble codes.  This will give you information about why the check engine lamp is on.  You'll need access to an OBD-II code reader to read the codes.  AutoZone and AAMCO have advertised they'll read the codes for no charge.  Once you have the code numbers, follow-up with me.  I'll tell you what they mean and advise on how to diagnose/repair the issue.  Chances are good that the cause of the check engine lamp is also the cause of your poor fuel economy.