Ageing or contaminated downstream oxygen sensor (Bank 1, Sensor 2) with a slow response; Exhaust leak near the sensor or upstream of it, letting in false air; Damaged, chafed or corroded sensor wiring or connector, including high resistance in the signal or ground circuit; Sensor contaminated by oil, coolant, silicone or fuel additives; Sensor heater circuit fault slowing the sensor's warm-up and response; Ageing or degraded catalytic converter or an underlying fuelling fault affecting the downstream signal.

P0139

O2 Sensor Circuit Slow Response Bank 1 Sensor 2.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
1: AreaFuel and air metering
39: FaultThe specific fault in that area

What it means

P0139 is the generic code for a slow response from the downstream oxygen sensor (Bank 1, Sensor 2), which sits after the catalytic converter. The control unit monitors how quickly this sensor's signal switches between rich and lean when the fuelling changes, usually during a deceleration fuel cut-off or a forced fuelling change. If the signal changes too slowly or lags behind what is expected, the code is set.

Symptoms

Engine management light on
Usually no noticeable change in driving
Slightly higher fuel consumption in some cases
Failed emissions test
Rough running or poor fuel trim only if there is also an underlying fuelling or exhaust fault

Possible causes

Ageing or contaminated downstream oxygen sensor (Bank 1, Sensor 2) with a slow responselikely
Exhaust leak near the sensor or upstream of it, letting in false airlikely
Damaged, chafed or corroded sensor wiring or connector, including high resistance in the signal or ground circuitpossible
Sensor contaminated by oil, coolant, silicone or fuel additivespossible
Sensor heater circuit fault slowing the sensor's warm-up and responsepossible
Ageing or degraded catalytic converter or an underlying fuelling fault affecting the downstream signalrare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting engine temperature, load and speed when the fault set.
Check for other codes, especially heater circuit, fuel trim, misfire or catalyst efficiency codes, and deal with those first.
Visually inspect the sensor wiring and connector for damage, melted insulation, water ingress and corrosion, and check the harness routing near the exhaust.
Inspect the exhaust from the manifold to the sensor for leaks, cracked joints or failed gaskets.
With the engine warm, view live data for the Bank 1 Sensor 2 signal and check it is stable and responds quickly when you snap the throttle open and closed; compare with the expected behaviour for that model.
Check the sensor heater circuit supply, ground and resistance against the manufacturer's specification.
Check short and long term fuel trims for signs of a fuelling fault, and look for oil or coolant consumption that could contaminate the sensor.
Check the wiring continuity and resistance between the sensor and control unit for the signal and ground circuits.

How to fix

Repair or replace damaged wiring, terminals or the connector.
Repair the exhaust leak, replacing gaskets or damaged sections as needed.
Replace the downstream oxygen sensor with a correct specification part if it is slow or contaminated.
Repair the cause of any contamination, such as an oil or coolant leak, before fitting a new sensor.
Fix any underlying fuelling, misfire or catalyst fault that is affecting the signal, then clear the code and confirm it does not return.

Can I keep driving?

Low severity

The car can normally be driven while you book it in, as this fault rarely affects driveability. Do not leave it for long, since it may hide other problems and can cause an emissions test failure.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.