My 2007 cobalt is wasting too much gas, what could it be? Plus it has the engine light on

Having the "Check Engine" light on will make the engine run rich and waste gas. You can get the codes read at the auto parts stores and they can tell what is causing the problem. Write down the code number and post it here. Someone can tell you what it takes to repair your car.

I dnt have a code but i was told it was the oxygen sensor, yet i already change both sensors and like in 2/3 days the engine light went on. Do the oxygen cables have to be original?

The O2 sensor cables do not have to be original but they do have to be connected correctly. The upper O2 sensor(s) controls the amount of fuel sent to the engine. The lower sensor(s) are after the cat and monitor the health of the cat converter.

how is the engine temps? normal? tires are at proper inflation? idling/stop-n-go can be bad too.

just because the code is for an oxygen sensor doesnt mean that that is the problem the engine could be running leaner than normal in turn the computer compensates for the lean (not enough fuel) and dumps extra fuel in to the engine to get back to where it should be hense more fuel comsumtion the o2 sensors may have been working correctly it could be another skewed sensor or vacuum leak or fuel delivery problem to the engine if u give us the code that will most diffinately help to understand what is happening

could also be the mass air sensor. if it is bad could think the air is at 0 deg. f. and be dumping more fuel into your motor

do you need the second o2 sensor or will the car run rich?

I have cylinder 4 misfire code p0304 I already changed the coil for that cylinder and still the same problem and burning more fuel than usual

My 2008 cobalt wasting too much gas already changed the spark plugs,air filter,gas filter have no check engine light on what can it be