The check-engine/service light has been on for years in this 2001 truck. 4.2 V6 engine, automatic XMSN, 2WD, 135K miles. It used to be my father's truck and he just covered the light with tape.
I was at an O'Reilly's yesterday and had them check for codes.
Their reader showed no codes of any type, and he could not clear the light...

What the heck is going on with that?

 

Have you tried disconnecting the negative battery terminal for about 30 minutes ?

 

More than once...

 

There is a specific routine for no codes outputted
You need a 2001 PCED
Your OBD2 self test connector may just have no power

 

What is a PCED?

 

It's a Ford repair manual book
Powertrain Control / Emissions Diagnosis
Here is a pic of a 1993 one