If your Ford Excursion recently had its battery replaced (or had a dead battery and it was charged) or had it's emission trouble codes reset, you will need to drive it for a few hundred miles in order to trigger the readiness tests to run and set the Oxygen Sensor monitor.
We recommend driving for at least 300 miles (1-2 weeks) in a variety driving conditions; traffic, freeway, city, stop and go. Getting the O2 monitor to set can be tricky.
If after this process the O2 readiness test fails to run, there are no trouble codes present (check engine light is not on), you will need to have Ford (or a well trained smog check mechanic) manually attempt to trigger the readiness test by following a very precise driving pattern utilizing the smog station's dynomometer. If trouble codes are present (which are stopping the readiness tests from running) those trouble codes will need to be diagnosed.