P0139 — OBD2 Diagnostic Trouble Code

This is a common Onboard Diagnostic-II code P0139 that indicates a voltage drop below in the O2 Sensor 2 Bank 1 circuit. During the deceleration cutoff point, the voltage should be at least 02 volts for at least seven seconds.
Essentially, the ECM (Engine Control Module) in your car is determining that the sensor’s reaction is too sluggish. This will start to set off the fault code, and the dreaded “check engine” light will appear on your dashboard.