Powertrain, generic · Fuel and air metering

P0138 O2 Sensor Circuit High Voltage 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. P0138 means the signal stays above the upper limit. A zirconia sensor can hardly produce more than about 1 V, so readings above 1.05–1.15 V point to an outside voltage such as the heater supply on the signal wire; GM also sees an open circuit here because of its bias voltage. Ford: above 1.1 V for 10 s; GM: above 1050 mV. Subaru again uses a functional test: the minimum voltage never falls below 250 mV within 200 s after fuel cut-off, so the sensor looks permanently rich. Monitored continuously, two trips. When it fails, the control module stops the downstream trim and cannot evaluate the converter; there is no limp mode.

Likely causes

Signal wire shorted to heater supply or battery voltage
Sensor faulty internally (heater-to-signal short)
Signal circuit open or high-resistance (GM bias logic)
Genuinely rich exhaust or oil/coolant contamination

Symptoms

  • Check engine light on (usually after two drive cycles)
  • Catalyst monitor readiness not set
  • Usually no noticeable change in driving

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.

What to check first

  1. Check fuel trims and fuel pressure
  2. Inspect wiring
  3. Replace sensor if contaminated

Parts involved

oxygen sensor bank1 sensor2oxygen sensor wiringcatalytic converter

Related codes

P0137P0140P0158P0144P0132P2271

References: law.cornell.edu, gsi.ext.gm.com, newtonnet.co.uk, fordservicecontent.com.