P0139 indicates the powertrain control module (PCM) has detected a slow response from the oxygen sensor on bank 1, sensor 2 (the downstream O2 sensor). The sensor is not changing voltage quickly enough as exhaust conditions change.
The sensor itself may be worn, contaminated, or have internal wiring failure, causing a sluggish voltage response.
Leaks before the sensor can introduce extra oxygen, while restrictions can prevent normal gas flow, both slowing sensor response.
Corroded pins, broken wires, or loose connectors can add resistance or intermittent signals, making the sensor appear slow.
If the heater circuit fails to bring the sensor to operating temperature, the sensor will react more slowly.
Rarely, the PCM may misinterpret sensor data due to internal failure, but this is far less common than sensor or wiring issues.
The definition is universal; the diagnostics are engine-specific. Open your vehicle for the codes documented for it:
It is possible but extremely uncommon. In most cases the problem is the sensor, its wiring, or an exhaust leak. PCM replacement should be a last resort after all other possibilities are ruled out.
No. P0139 specifically refers to the downstream sensor on bank 1. The upstream sensor (Bank 1 Sensor 1) is monitored by a different code (P0130‑P0134). Only replace the sensor that is proven faulty.
Cleaning may help if the sensor is merely contaminated, but most O2 sensors are sealed units that cannot be reliably restored. Replacement is usually the recommended solution.
A slow response means the sensor is sending a voltage signal, but it changes too gradually as the exhaust composition varies. This can be caused by aging sensor elements, heater issues, or compromised wiring.