P0138 Honda Civic
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2) · Powertrain Code · Medium · Repair: Easy
What is P0138?
High voltage from the downstream O2 sensor on bank 1.
When the Honda Civic 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 Honda Civic.
This repair is rated easy, meaning basic tools, no lift needed.
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 Honda Civic.
Common Causes on Honda Civic
- Downstream O2 sensor reading high voltage (stuck rich signal)
- Short in O2 sensor wiring to voltage source
- Fuel system running too rich — leaking injector or high fuel pressure
- Silicone-contaminated O2 sensor from RTV sealant use
Symptoms
- Check Engine Light on
- Poor fuel economy from PCM compensating for false rich signal
- Possible rough idle
- Black exhaust smoke in severe cases
Notes for Honda Civic
Silicone from aftermarket gasket makers can permanently poison O2 sensors — always use Honda-approved sealants or OEM gaskets on exhaust components.