1996 Ford Contour check engine light is on and I checked th

First parts to replace are the 02 sensors, these are made to give 100,000 miles of service and you are a tad over, P0135 for bak1 sensor1 is the first 02 sensor on the #1 cylender side, I recommend replacing ALL 02 sensors at this point, P0420 we hope is being set by lazy 02 sensors, your car has 2 02 sensors one befor the converter and one after, these should switch quickly one the front sensor and remain almost flat one the back sensor, If 02 readings are not right, you will get poor fuel mileage, and could be the reason for the efficiency code too.