MPG always drops with a DTC because ECM puts car in default mode. Just like when you first start your car, no sensors are read so no fuel trim is done. Older OBDII readers won't read the Cobalt codes because it needs CANDI protocol.
If the OBDII scanner was made before the chassis initial dates, then the scanner will do a general OBDII scan, not vehicle specific. Ensure that on your plug and play socket pins 4,5,and 16 are not bent or missing, that is vital for communications from Engine Control Module to Scan Tool.
When you first start your car, the CEL lights up and does a quick check of all sensors and solenoids (inputs and outputs), except for the O2 sensors (they require a minimum temp of 600 degrees and time in order to achieve closed loop). Your car is not in default mode, its called "fault" :P
Not saying your wrong Coby7, just thats the information UTi, Wyotech, and technician schools say.
Originally Posted by WISHiBOUGHTanSS
woah woah.... cel's can change the way your car drives? i thought it was just like a "hey you messed something up" alert.
Yes, once your car's computer has found a fault or bad reading from one of many sensors it monitors, it goes into what is called Keep Alive Memory. It basically stays in that mode until the problem is fixed, codes are cleared, and the ECM is detecting normal operations. I saw up here about an 02 sensor and that is quite common for today's cars, but dont always rule it out as a first thing. Your vehicle will lose its optimal performance (fuel economy and power) in order to keep that car running, if it were an 02 sensor, it will trigger to spike up the air/fuel mixture from the 14.7 (stoichometer) to (rich). Not saying this is horrible, but it will cause if ignored, serious wear and tear (spark plugs may foul up, flood in combustion chamber, excessive fuel thrown into catalyic converter, there may be more too) on the engine.