Check Engine Light Turn On In My Explorer 1991 Xl

I have a Ford Explorer 1991. I brought to Chile in 1992 after finalizing my stay in Golden, Colorado, USA. It has been a wonderful vehicle for me and my family, but since two years started to have problems. I think that the problems started when the gasoline consumption started to rise to the double approximately. The main problem is that the check engine lights turns on after driving a few kilometers. If I stop the engine and turn on again, it will turn on the light very soon. I remember that the 2012 technical revision was not pass by the vehicle because of gas emission problems. My car has to have three gas emission test a year because catalytic cars fabricated before 1994need to accomplish that. In Santiago, Chile, where I am living now, I have gone to several garages. All of them and even the Ford dealers garage doesn't have the scanner for my "old "vehicle as they say. In the places where they have been able to scan the vehicle, the problems is attributed to oxygen sensor. This has been changed several times and the problems still remains. I have changed severals parts of the car: Catalytic, mass air flow, thermostat and temperature sensor. ¿Could you please to help me?. ¿Is there someone (mechanic or Ford representative in Santiago, Chile who can provide me the solution? Regards, Hugo Vieytes