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 amount of air and fuel coming out of the catalytic converter to ensure the catalytic converter is functioning properly. If the voltage is high (approx. .9V) there is an excess of fuel in the mixture. If the voltage is low (approx. .1V) then there is an excess of air in the mixture. 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 a high voltage (steadily above .9 volts) for more than 10 seconds indicating a lack of oxygen in the exhaust stream and an abundance of fuel at sensor 2 on the bank 1 of the engine.

What Are the Symptoms of Code P0138?

• Check Engine Light
• Engine runs rich
• Lower fuel economy
• Rough idle
• Strong fumes

What Is the Cause of Code P0138?

• Faulty O2 Sensor
• Short to battery voltage in O2 sensor signal circuit
• Corroded wiring
• Fuel pressure too high
• Engine coolant temperature sensor

How Serious Is Code P0138? – 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 damage the catalytic converter(s).

Code P0138 Common Diagnosis Mistakes

Replacing the O2 sensor without checking to see if the vehicle is running rich due to another problem.

Tools Needed to Diagnose Code P0138:

• Tools Needed to Diagnose:
  • FIXD
  • Multimeter
  • Fuel pressure gauge

How to Diagnose and Repair Code P0138:

Difficulty of Diagnosis and Repair – (2 out of 5)

1. Check the wiring at O2 sensor for damage; if damaged, repair or replace affected harness.
2. Check voltage at the O2 sensor; if the voltage is steadily high (.9V or higher) the O2 sensor may be faulty. Before replacing, check the following:
   1. Verify coolant temperature is reading properly (consult vehicle manual for specification)
   2. Verify fuel pressure is within spec.
3. If the O2 sensor is reading high voltage and the fuel pressure and coolant temperature are within spec, replace the bank 1 downstream O2 sensor.

Estimated Cost of Repair

For error code P0138, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair.

• Oxygen sensor $200-$300
• Fuel pressure regulator $200-$400
• Engine coolant temperature sensor $150-$200