my question deals with the 4x4. it will no longer ingage. on 2002 Dodge Ram 1500

its a 2002 ram 1500, 5.7 liter hemi with 150,000km. The service 4x4 light has been coming on and off and when it is on the selector switch will not ingage the 4x4. I had it scanned and no codes were found. And as of today the light has decided to stay on leaving me with no 4x4. any ideas?

Asked by for the 2002 Dodge Ram 1500