DTC's only pop up if a sensor has gone bad, or if the sensor is reading out of spec data. That said, there are vast numbers of systems and problems which are not monitored by your OBD1 computer.
You should be getting 19 to 20 MPG in combined highway and city driving. If it is all city driving you are probably looking at closer to 15 MPG as an average, so you arent too badly off at 12 MPG.
That said I would make sure your tires are fully inflated. I would also do a full tune-up (plugs, wires, cap, rotor, oil change, trans fluid and filter, fuel filter, air filter, PCV valve, and have your injectors and IACV cleaned)
After you do that, track your fuel economy again over a few weeks. I can almost guarantee that will solve it, but if it doesnt, at least you know the very basics of the car are solid and you have a good place to work from. (Similar to checking good ground before you test electrical systems)
The 3 most common reasons for poor fuel economy are improperly inflated tires, a dirty air filter, and a bad O2 sensor (and in the case of the O2 sensor, you should have a CEL on, which you dont, so it is extremely unlikely it is bad)
If it isnt any of that you can start looking at the Catalytic Converter, the Mass Air Flow Meter, the electronic choke, stuck break calipers, horribly dirty injectors, a bad ECU, etc... however as you go down the list, the probability of that being a problem, and not having other problems as well, also decreases.