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EmissionsSeverity Low: usually safe to drive

P0139O2 Sensor Circuit Slow Response (bank 1 sensor 2)

What it means

The oxygen sensor bank 1 sensor 2 switches between rich and lean more slowly than the ECU expects. The sensor is usually aged or contaminated. The downstream sensor (after the catalyst) is mainly used to check how well the catalyst works.

Symptoms

  • Check engine light
  • Usually no change in how the car drives
  • Catalyst monitor may not complete

Common causes

  • Aged or contaminated oxygen sensor
  • Exhaust leak near the sensor
  • Heater circuit fault keeping the sensor too cold
  • Engine burning oil or coolant

What to check first

  1. Scope the sensor at 2,500 rpm: rich-to-lean transition should take under about 300 ms
  2. Check the heater circuit
  3. Check for exhaust leaks and signs of oil or coolant contamination
Can I drive with P0139? Usually yes. P0139 rarely affects how the car drives, but it keeps the check engine light on and can fail an emissions test.

Generic OBD-II meaning. Manufacturer-specific meanings and checks can differ; always check the service information for the car.

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P0139: questions and answers

What does P0139 mean?

P0139: O2 Sensor Circuit Slow Response (bank 1 sensor 2). The oxygen sensor bank 1 sensor 2 switches between rich and lean more slowly than the ECU expects. The sensor is usually aged or contaminated. The downstream sensor (after the catalyst) is mainly used to check how well the catalyst works.

What are the symptoms of P0139?

Check engine light. Usually no change in how the car drives. Catalyst monitor may not complete.

What causes P0139?

Aged or contaminated oxygen sensor. Exhaust leak near the sensor. Heater circuit fault keeping the sensor too cold. Engine burning oil or coolant.

How do you diagnose P0139?

1. Scope the sensor at 2,500 rpm: rich-to-lean transition should take under about 300 ms 2. Check the heater circuit 3. Check for exhaust leaks and signs of oil or coolant contamination

Can I drive with P0139?

Usually yes. P0139 rarely affects how the car drives, but it keeps the check engine light on and can fail an emissions test.