Does P0139 mean my cat is bad?

No, P0139 is specifically the sensor responding slowly. Replace the sensor first before worrying about the cat.

How often should downstream O2 sensors be replaced?

Typically every 80,000-120,000 miles, or when codes appear.

Low Severity

Emissions System

The downstream O2 sensor on bank 1 responds too slowly. This can mask catalytic converter degradation.

Common Fix

Replace downstream O2 sensor

Estimated Cost (DIY)

$25 - $80

Aging O2 sensorlikely

Contamination from oil or coolantpossible

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Does P0139 mean my cat is bad?: No, P0139 is specifically the sensor responding slowly. Replace the sensor first before worrying about the cat.
How often should downstream O2 sensors be replaced?: Typically every 80,000-120,000 miles, or when codes appear.

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