1998 Forester with Engine Code P0139

[Sarahfehr93]

Does anyone know what this engine code means? P0139 O2 circuit slow response.

Thanks!

 

[Ecmbuster]

https://www.engine-codes.com/p0139_subaru.html
or
https://obd2pros.com/dtc-codes/p0139/

The engine controller is expecting an output from the Oxygen Sensor.
A vehicle that has many Km or miles will suffer O2 sensor failures for many reasons and the sensor is not repairable.
A scan tool can measure O2 sensor activity via voltage output. An oscilloscope can also measure its output.