Check the following items:
1. good clean air filter?
2. no leaks in the air filter box, cover closed properly?
3. connecting hoses attached to the air box properly?
4. pcv valve working correctly?
1. Change the air filter if you don't recall the last time it was done. I record the mileage on filter with a permanent marker. Put it in a location where its visible with the air box opens. Change the air filter every 2 years or 30000 miles which ever occurs first.
2. Ensure that the two parts are aligned properly and that all the clamps and hoses all fit correctly. A good way to find vacuum leaks is to use vaseline on the ends of any poor fitting hoses. Do one at a time and you can isolate the problem hoses.
3.The two vent hoses that attach the valve covers to both sides of the air box can become loose with age. Adding some screw clamps can solve a small leak. Replace the hoses if they are hard and dried out. Also inspect the throttle body connection part of the air box for loose clamps and fitment.
4.Pcv test is pretty simple. Start the engine, open the oil cap, then hold a piece of paper over the cap. If the pcv is working the engine vacuum should try to suck the paper in. If the vacuum is weak then clean or replace the pcv valve.
If the all the above checks are good then its time to replace the front oxygen sensor.