I think he should wait until he knows that his actual mpg is bad before replacing a costly O2 sensor or coolant temperature sensor.
If either of those were defective, they would trigger a warning, which he has not reported.
And since the coolant sensor likely reports to the temperature gauge through the ECU because the needle is buffered, a sensor problem would show up as low temperature on the gauge, which he has not reported.
If there is no record of air filter or plugs being changed, they should be looked at regardless. I had no records on my car, but also no problems. My air filter looked OK so I left it in. I looked at my plugs and they looked OK but were the wrong kind. They were the popular NGK BKR6EIX single electrode Iridiums, which NGK emailed me had the wrong resistance and tip (firing position) for the 1.9L. They recommended the dual electrode NGK BKR6EK, which I installed.
Last edited by vintage42; 12-20-2012 at 06:51 PM.