2004 Mazda 3 • 80 miles

I recently noticed that I was getting about 1 MPG less for mixed driving and so I figured it W's time for a tune-up. I had a faulty pcv valve replaced, new spark plugs, new coil connector, and oil change. Now, I get 2 MPG less on mixed driving and 5 MPG less on highway. I think the mechanics are honest, but I think they missed something. Can you give me some thoughts on what to have them check?
October 25, 2012.

There does not seem to be a trust issue here, as far as with your chosen mechanic. I dont perceive any wrongdoing, and there may be factors not included in the words in your post. Most car manufacturers use adaptive systems and it may simply be a matter of allowing the car to adapt. 2 mpg difference by itself is not alarming. I would look at the additive and multiplicative fuel trim and see if it is at or near its limit. There are many factors to consider, lets take them one at a time, agreed?

Dr. Hagerty
Oct 25, 2012.
I don't see your location but I suspect the weather is cooling off which will reduce the fuel mileage. Oxygen sensors get clogged and becomes less sensitive and accurate which will also effect your mileage.

Oct 25, 2012.
I disagree. I think 5 mpg on the highway is significant. That said, I have a previous mechanic, and he supported your suggestion that their might be issues with my fuel. I disagreed until, my buddy suggested that different formulations with the cooler weather can create problems. I took his advice, and ran a can of Seafoam on a quarter tank of gas, and then, I've been running premium fuel from Exxon. I appear to be back to my previous mileage. As for the O2 sensor, I asked them that, but they said it checked out fine. Thanks, for the input.

Nov 9, 2012.