As your Oxygen Sensor ages, it slowly falls out of calibration. It will produce less voltage, and the rate that it can respond to changing exhaust content will slow significantly (slower swing rate). However, if you monitor the output of the O2 sensor, it will still look normal since the fuel injection computer has richened the mixture to get the desired output of the O2 sensor.

The service interval for a typical 3-wire (heated) Bosch oxygen sensor is 60,000 miles, and you'll start to notice the fuel mileage get worse and worse after 100,000 miles. Replacing your Oxygen Sensor will pay for itself in increased fuel economy.

One other thing to check is the air box thermostat to make sure that it isn't drawing a continuous draw of hot air from the exhaust manifold. The constant supply of hot air on a warm day will kill an Air Mass Meter (which will cause the fuel system to run at a full-rich mixture in "Limp Home" mode).

God bless,
Fitz Fitzgerald.