P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

What is P0138?

Diagnostic Trouble Code P0138 (DTC P0138) is defined as “O2 Sensor Circuit High Voltage (Bank 1 Sensor 2).”

Oxygen sensor 2 is a downstream oxygen sensor or the secondary heated oxygen sensor (HO2S 2) on Bank #1. It measures the air-fuel mixture or the oxygen storage capacity of the catalytic converter and produces a relevant output signal (normally about 450 mV). 

A triggered P0138 code means the Engine Control Module (ECM) has detected a high voltage signal (over 999 mV) from the downstream sensor for more than 10 seconds. 

A high voltage signal indicates an imbalance in the air-fuel ratio — a low oxygen concentration in the exhaust gas (and an excess of fuel running through the engine).

Common symptoms

The P0138 code comes with some noticeable symptoms. Here’s what you may experience when your vehicle has a high voltage HO2S 2 signal: 

• Engine Running Rich: P0138 means that there’s excess fuel (less oxygen) running through your engine, i.e., the engine’s running rich. This leads to low fuel economy and incomplete combustion, which can cause engine performance problems.
• Rough Idle: The engine may vibrate heavily due to the combustion of an imbalanced air-fuel mixture, resulting in rough idling. Moreover, to compensate for the high fuel pressure, the engine may run lean (with a high air fuel ratio) during sensor testing — leading to misfires or stalling.
• Heavy Exhaust Fumes: When you’re driving, or even if the car is idle, you’ll notice heavy exhaust gas emissions. This is because a faulty O2 sensor can lead to an improper air-fuel ratio and incomplete fuel combustion. These emissions will also have a strong odor.
• Blinking Check Engine Light: When the engine control module stores code P0138, you’ll immediately notice a Check Engine Light or MIL illumination (Malfunction Indicator Light).
  

Can I still drive?

Yes, it’s possible to drive with engine light code P0138. However, as it’s a moderately severe code, it’s best to stop driving once you detect it.

In fact, extended driving with this code can cause:

• Internal engine damage
• Catalytic converter failure
• Fouled spark plugs
• Reduced fuel efficiency 
• More pollutants in emissions

Additionally, you won’t be able to tell if other engine light codes are popping up while you leave this issue unchecked. So, addressing the Check Engine Light issues as soon as possible could help save your vehicle from further damage and costly repairs.

P0138 causes

Here are common causes behind code P0138’s high HO2 sensor voltage:

• Malfunctioning or damaged O2 sensor (or an old sensor with more than 100,000 miles)
• Rear sensor heater circuit malfunction
• Short circuit in O2 sensor signal circuit
• High battery voltage
• Corroded harness or damaged wiring
• High fuel pressure (or a defective fuel pressure regulator)
• Fuel injector leaks or engine vacuum leaks
• Exhaust leak
• Ignition misfires
• Failing engine coolant temperature sensor
• Mass airflow sensor failure
• Manifold air pressure sensor failure 
• Inefficient or malfunctioning catalytic converter (rare)
• Engine control module or powertrain control module issues (rare)

Diagnosis

Here’s how a mechanic would diagnose a P0138 issue:

1. Check the codes using a scan tool, document the freeze frame data, and clear the codes to verify the issue. 
2. Check the O2 sensor for damaged wiring, a loose connector, or wiring harness corrosion. 
3. Test the O2 sensor for low or high voltage and compare the sensor voltage data of sensors 1 and 2. They should have nearly identical sensor voltage, but sensor 1 should have a higher oxygen level reading. 
4. Check the O2 sensor for physical damage or fluid contamination from engine leaks. 
5. Test the engine coolant temperature sensor to see if it’s functional. 
6. Test the fuel pressure regulator to see if it’s within manufacturer specifications and check for possible high fuel pressure (a low air fuel ratio). 
7. Check if there’s an exhaust leak before the sensor and if the gas cap is loose. Also, ensure that the catalytic converter isn’t clogged or damaged. 
8. Follow other pinpoint tests for Bank 1 Sensor 2 issues specified in the manufacturer’s manual.

Note: Code P0138 could result from several issues. So, it’s best to let a mechanic conduct a diagnosis before you replace any parts, like an old sensor.

Possible repairs for P0138 & Costs

Here are the actions your mechanic would take to fix DTC P0138:

1. Repairing electric circuits or replacing connectors if the wiring harness or electrical connections are damaged, corroded, or loose. 
2. Repairing the O2 sensor signal circuit in case of a short due to high battery voltage. 
3. Replacing the engine coolant temperature sensor and fuel pressure regulator if they’re malfunctioning. 
4. Replacing the downstream oxygen sensor (rear sensor) on Bank 1 if faulty. 
5. Replacing a leaking fuel injector or damaged catalytic converter. 
6. Repairing the exhaust leak, if present. 

Once the repairs are done, your mechanic will verify if the Check Engine Light is still flashing.

Note: The repair costs will depend on the diagnoses, your mechanic’s labor rate, and the repairs needed.

A P0138 diagnosis could cost between $75-$150. After the diagnosis, your mechanic will give you a better idea of the cost of repairs.

That being said, the estimated costs for fixing the required parts (including the cost of labor and the replacement part) are:

• Engine Coolant Temperature Sensor: $150-$200
• Oxygen Sensor (Downstream Sensor): $200-$300
• Fuel Pressure Regulator: $200-$400