Vehicle has 200,000 miles runs and looks great yet fuel economy seems low (12 miles per gallon). I have had many 1/2 3/4 ton chevy's in the past and was always able to get at least 16 miles to the gallon.

My question is could it be the oxygens sensor that is causing my gas mileage to go down? I have changed the air filter, cleaned injectors, and as I have said it seems to run good but rich. I know that if the oxygen sensor is running in open loop it will run rich yet shouldn't it set off the service engine light if it is?