Hello XXXXXXXXXXXXX, the code p0138 is not shown in the Ford information,it must have been pulled with a generic scanner, all information for that code shows it is for the down stream sensor, not the sensor 1 that you stated in your question,sensor one {upstream is before the cat,sensor 2 [downstream] is the one behind the cat. I would check the wiring and make sure the connection is clean and tight and you have no exhaust leaks that can allow air to be sucked in and effect the sensor reading. Then I would take care of the code p0301, the miss on cylinder #1, since the miss can affect the o2 sensor readings. When the miss is repaired I would clear the codes,and re try the car,if the p0 138 returned I would replace the down stream[rear] sensor.
Auto Mechanic
Auto Mehanic 39 years,30 with the NYPD Fleet Service Division