One problem related to o2 sensor has been reported for the 2004 Ford Explorer. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 Ford Explorer based on all problems reported for the 2004 Explorer.
Purchased a 2004 Ford Explorer in June 2005. In 2006 had to replace the O2 sensor. In may 2007, had to replace the same O2 sensor. Now in January 2008, was driving and truck just stopped on a hill with traffic coming up, large diesel truck, almost causing an really bad accident. Had vehicle checked and the exact same 02 sensor is out for the 3rd time and was told this was not normal and that there is a large number of 2004 vehicles with the same problem. Was told this by 2 different dealerships in two different states. Contacted Ford and was pretty blown off and stated that because of the mileage on the vehicle, there was nothing that could be done because I had an outside warranty and not a Ford warranty. They didn't even contact the dealerships after claiming to have done a investigation. Was told I just had to pay again. Now what if this happens again and I'm not so lucky that a big diesel truck can whip into the other lane to avoid hitting me.