There are two scenarios which would cause your Ford Explorer to fail the functional portion of the smog check which inspects for any Malfunction Indicator Lamp trouble.
In the first possibility, via the OBD II Data Connector Link, the smog machine may detect an active, pending, hard or soft OBD-II trouble code during the smog test; causing a smog check failure. It is not necessary for the Malfunction Indicator Lamp to currently be illuminated in this scenario. The MIL light may have been illuminated recently, and have gone undetected. If the light fails to return, and upon 3 drive cycle completions, the pending or soft trouble code should disappear; allowing your Ford Explorer to pass the smog check.
If the MIL light does illuminate over the course of the next 2 weeks, a "hard fault" exists and should be diagnosed by a smog check repair station. The station will retrieve the trouble code(s) from your Ford Explorer's ECU and inspect the problem further.
In the second scenario your Ford may fail for Malfunction Indicator Lamp trouble if the smog machine detects certain required Emission Monitors (also known as readiness flags) are "Not Ready" or "Not Complete".
In order to "Ready" or "Complete" the required emission monitors the OBD II system must complete at least one good drive cycle. A Drive Cycle is a sequence of internal tests which the OBD II computer runs while you are driving your Ford Explorer. These self tests insure all emission systems are functioning properly. Emission monitors are erased often on vehicles which have had recent repairs requiring disconnecting of the battery, and/or the emissions computer. Disruption of 12+ power to the ECU clears all readiness flags. These vehicles will need to be driven in order to complete the required monitors.