I bought my 2007 Mazda 3 2.0L a few months back from a reputable dealership. A month after I got it I had to replace my thermostat, no big deal. I have an extended warranty on the car. Another month goes by and I need an oil change as I drive the car about a hundred miles a day on the interstate. About a month and a half goes by and I notice I'm pretty low on oil. (I'd say somewhere between 1000 and 2000 miles later) I think maybe that's just normal, so I throw in a quart and a half of oil. shortly (a week, maybe two) thereafter I hit a deer. Again no big deal because I have full coverage and it was mostly body damage. The car is good as new. Great!

Its been 2 months since the accident. 2 days ago my CEL came on. I had my local autozone read the codes for me. Its a bad O2 sensor (Bank 1, Sensor 1). My dad and I took a look at the sensor. It wasn't plugged in right. We pushed it all the way in and reset the CEL. I took the car out for a spin to see if the light comes back on. I notice the car is idling funny, the gears are shifting weird, and its stuttering. I pull up to a stop sign a block away from my house... wait for some cars to pass, and as soon as I hit the gas and the engine shut down. I called my dad and tried to start it a couple of times, on the third try it sprung to life. No CEL at this time. We to it to my garage and checked the other sensor, it was fine. I knew I was due for another oil change already. We checked the oil and it was BONE DRY. This is after the remaining 3000 miles plus some change. So I took another car and got some oil. We put 2 quarts in this time. The engine was still running funny so we called the dealership.

Once we were at the dealership they estimated the cost for the parts. I needed a new O2 sensor and a tail light. (The Tail Light is unrelated, its just getting moisture in it). The warranty wouldn't cover either part so they just did an oil change even though I just put new oil in an empty engine. They also told me to come back in 500 Miles to see how much oil the car has used.

Since then I have replaced the O2 sensor, no CEL yet.

So here are my questions:

1) Why does my engine use up so much oil in just 5000 Miles?
(Note: There are no leaks and theres not black smoke coming out of the tail pipe. I also checked the coolant and didn't find the oil there as that has been mentioned to me before. I haven't checked around the spark plugs, mainly because I am not 100% sure where they are and my owners manual is USELESS.)

2) I've noticed a pretty big drop in fuel efficiency as well, why might this be?
(I use to get 500 Miles to the tank, now I'm getting 350 if I'm lucky.)

I don't always get around to maintaining my car. I try to make sure everything is In check at least one a month, like fluid levels and such. I am also almost always inspecting the outside of my vehicle and I do turn down the subs on the highway from time to time to make sure the engine sounds okay. I go to a commuter college 50 miles away full time and the other part of my full time I'm at work. I would like to try to figure this out sooner rather than later.

If you have any questions for me I'll be sure to answer them.

 

Hi, hi45689!

You do a lot of miles.
First thing, you already know that you don't do enough maintenance. But then you also need to do the right maintenance.
It would be helpful to know what the actual digital trouble code was, because there is no code for "bad O2 Sensor". The O2 sensor (HO2 sensor to be correct) is a collector and messenger for data that the computer uses to calculate the proper fuel trim. If these calculations go out of whack you get a code and CEL. It could be the sensor but most likely there are other reasons and cheaper ones to fix. So don't kill the messenger in a knee jerk reaction.

Normally at this point I suggest to get a decent tune up: new spark plugs and Coil on Plugs (CoP).
New air filter, new PCV valve, clean MAF sensor, check for vacuum leaks.

This PCV valve may be the reason for the oil consumption. If it does not work correctly oil gets sucked into the intake and burned off.

It is also possible that the piston rings are worn. A compression test and a leak down test would reveal this.
And it is possible that the valve guide seals are worn.

However, I worry that the engine was damaged when it was low on oil. That is why I would want the compression test and leak down test done first. If you need a new (used) engine then it is not worth throwing money for parts at it.

You may not see blue smoke because the catalytic converter is burning off the oil fumes.
Black smoke would be caused by excessively rich fuel/air mixture.

Let us know how many miles are on the car. I would also like to know what kind of oil you are using.

I'll be looking for your response.