P0139: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)

What P0139 means

P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 1, places it in the fuel and air metering subsystem; the last two digits identify the specific fault.

Oxygen sensors measure leftover oxygen in the exhaust. Sensors before the catalytic converter (sensor 1) are used for closed-loop fuel control; sensors after it (sensor 2, and sensor 3 where fitted) are used mainly to judge how well the converter is storing oxygen. A conventional zirconia sensor switches between roughly 0.1 V (lean) and 0.9 V (rich).

Bank 1 is the side of the engine that contains cylinder 1. Inline engines have only bank 1, so a bank 2 code on an inline engine usually indicates a manufacturer-specific use.

Sensor 2 is the one downstream of the first catalytic converter.

What “Slow Response” means here

"Slow response" means the sensor takes too long to switch between rich and lean. The usual cause is an aged or contaminated sensor — silicone, oil ash or coolant can coat the element — though an exhaust leak near the sensor can also dull its response.

What you may notice

Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0139:

• Often no drivability symptom
• Higher fuel use if an upstream sensor is affected
• Catalyst efficiency codes stored alongside
• Failed inspection where OBD readiness is checked

Where to start diagnosing P0139

1. Identify the sensor: bank 1 is the side of the engine with cylinder 1; sensor 1 is upstream of the converter.
2. Look for exhaust leaks ahead of the sensor, which draw in air and skew readings.
3. Inspect the connector and harness for heat damage and contact with the exhaust.
4. Watch the sensor live: an upstream sensor on a warm engine should switch steadily; a downstream sensor should be comparatively steady.

Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.

Live data to watch

These standard OBD2 values relate to P0139. Support varies by vehicle; a value your car does not report will simply be missing.

Related codes

Codes from the same oxygen (O2) sensor family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.

Further reading

• It failed the emissions test
• Your fuel economy fell off a cliff
• O2 sensor switching and waveform analysis
• Fuel trim mastery: reading STFT and LTFT

Frequently Asked Questions