Powertrain, generic · Fuel and air metering

P0139 O2 Sensor Circuit Slow Response Bank 1 Sensor 2

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

The oxygen sensor after the catalytic converter on bank 1 (sensor 2) is a switching sensor (about 0.1–0.9 V) used for converter monitoring and fine mixture trim. P0139 means the sensor reacts too slowly to mixture changes. Subaru measures it around fuel cut-off: rich to lean from 500 to 300 mV must take no more than 0.51 s, lean to rich from 300 to 500 mV no more than 2 s; two drive cycles set the MIL. CARB requires such a test because a sluggish downstream sensor could let a worn converter pass. Since about 2009–2011 GM and Ford report this as direction-specific codes P013A–P013F instead; P0139 remains in use on older, Asian and Jaguar Land Rover systems. When it fails, the control module stops the downstream trim and cannot evaluate the converter; there is no limp mode.

Likely causes

Sensor aged or contaminated (oil ash, silicone, additives)
Exhaust leak ahead of the sensor
Sensor heater weak, sensor too cold
High-resistance ground or connector

Symptoms

  • Check engine light on (usually after two drive cycles)
  • Catalyst monitor readiness not set
  • P0420 may follow

Repair estimate

DifficultyEasyDIY possible
Parts and labour€60–250
Time0.3–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

oxygen sensor bank1 sensor2oxygen sensor wiringcatalytic converter

Related codes

P0140P0159P0145P0133P013AP0420

References: law.cornell.edu, saabwisonline.com, saabwisonline.com, static.nhtsa.gov.