Error code P0138 is described as O2 Oxygen Sensor Circuit High Voltage Bank 1, Sensor 2. This indicates there is a failure to have a low voltage output below 1.2 volts or a time period of more than 10 seconds, which signifies no oxygen in the exhaust. The O2 Sensor could be functioning improperly, or there could be a lack of continuity in the wiring to the ECU, or it is possible that power is not supplying sufficient enough voltage to the sensor.


The O2 Oxygen Sensor Circuit High Voltage Bank 1, Sensor 2 is normally located in the catalytic converter, before the muffler, in the exhaust system. This sensor monitors oxygen levels in the exhaust stream. This gauges proper fuel efficiency as well as checks that the engine is maintaining the proper fuel/air ratio for a clean burn.

Common Symptoms

  • Engine runs lean
  • Engine may lag or miss
  • Fuel efficiency suffers
  • Rough idle
  • Engine dies
  • Strong fumes emitted

Possible Causes

  • Defective O2 Sensor/air fuel mixture sensor
  • Defective wiring
  • Connection to the oxygen sensor could be faulty
  • High fuel pressure
  • Corroded wiring
  • Possible short

How to Check

In order to check the O2 Oxygen Sensor Circuit High Voltage Bank 1, Sensor 2, the scanner will show the P0138 code, which indicates the error, and the following should complete the checklist:

Corroded Terminals

Check connection to O2 sensor for corrosion as well as wire degradation

Fouling of Sensor 2

Fouling would indicate a rich fuel mixture of more fuel than oxygen. This could cause fouling of the O2 sensor. Oil or coolant from an engine leak making its way into the exhaust system could be another cause for fouling.

Electrical Connection Problems

Check for damage to exhaust pipes and catalytic converter. Check wiring harness for melted wires especially near hot exhaust parts.
Do a visual check of the connections as well as using a Volt Ohm meter for wire connections.

Other Causes

Another cause could be a damaged or clogged catalytic converter.
A leaking fuel injector could possibly be another cause.

How to Fix

The number one most common repair for this problem is replacing the O2 Sensor itself. Another repair would be to replace the wiring or connection for Bank 1, Sensor 2. If the catalyst is the problem, replace the catalytic converter. Replace the fuel injector if it is leaking.

Parting Tips

After repairs have been made, clear stored codes from the ECU and test drive the vehicle between 30-35 miles per hour for about two miles. If the Oxygen O2 Sensor signal ever goes to a negative voltage, or in excess of one volt, this is indication that the sensor is bad. These inconsistent readings are often the reason to believe the circuit is losing voltage or ground into the O2 Sensor circuit. If the O2 Sensor body has a chalky appearance, this indicates lag between phases and should be replaced.