P0138 Toyota Highlander
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2) · Powertrain Code · Medium · Repair: Moderate
What is P0138?
High voltage from the downstream O2 sensor on bank 1.
When the Toyota Highlander triggers code P0138 (o2 sensor circuit high voltage (bank 1, sensor 2)), it typically points to an issue in the powertrain system. Below you'll find the most common causes, symptoms, and repair options specific to the Toyota Highlander.
This repair is rated moderate, meaning some experience and specialty tools required.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your Toyota Highlander.
Common Causes on Toyota Highlander
- Downstream O2 sensor stuck high voltage — sensor internally shorted
- Short to battery voltage in O2 sensor wiring
- Fuel injector leaking causing rich exhaust reaching downstream sensor
- Saturated catalytic converter passing unburned fuel to downstream sensor
Symptoms
- Check Engine Light on
- Possible rich running condition
- Sulfur smell from exhaust
- Failed emissions test
Notes for Toyota Highlander
A stuck-high downstream O2 sensor often indicates the catalytic converter is being overwhelmed — fix any upstream rich condition before replacing the sensor.