P0138 – Meaning, Causes, Symptoms, & Fixes
Code P0138 Definition
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
What Does P0138 Mean?
Oxygen sensor 2 is the downstream oxygen sensor in relationship to the catalytic converter. It measures the air-fuel ratio coming out of the catalytic converter to ensure the catalytic converter is functioning properly. The catalytic converter works to maintain the stoichiometric air-fuel ratio 14.7:1 while the powertrain control module constantly flip-flops between rich and lean air-fuel mixtures due to the input from the upstream oxygen sensor (sensor 1). Therefore, the downstream oxygen sensor (sensor 2) should produce a steady voltage of approximately 0.45 volts. When trouble code P0138 is set, this indicates that there is high voltage (above 1.2 volts) for more than 10 seconds indicating a lack of oxygen in the exhaust stream in Bank 1.
- Check Engine Light
- Engine runs lean
- Lower fuel economy
- Rough idle
- Strong fumes
- Faulty O2 Sensor
- Short to battery voltage in O2 sensor signal circuit
- Corroded wiring
Code P0138 Severity – Moderate
It is okay to drive a vehicle with P0138 for a short period of time, but driving with this code for an extended period of time can overheat the engine and cause internal engine damage.
Code P0138 Common Diagnosis Mistakes
Replacing the O2 sensor without checking any wires or clogged/damaged catalyst.
Difficulty of Diagnosis and Repair – (1-4)
- Check around the O2 sensors for visual possible corrosion that might need to be cleaned.
- Check for damage to exhaust pipes and damage to the catalytic converter.
- Check live data from all 4 terminals of O2 sensors. Make sure your key is on and your engine is off when checking this. You are checking for ground and signal reference. As long as you are getting ground and .3 volts of signal reference.
- If these steps do not clear your Check Engine Light, you may need to visit a mechanic because there may be internal electrical issues.