P0138 fault code causes, symptoms, repair

What does the P0138 code mean?

The P0138 is a generic OBD-II powertrain trouble code. Its official definition is “O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)“.

This diagnostic trouble code is stored by the vehicle’s Engine Control Module (ECM) when it detects that the signal voltage from the downstream oxygen sensor (Bank 1, Sensor 2) is persistently and abnormally high. “Bank 1” refers to the side of the engine containing cylinder number 1. “Sensor 2” is the post-catalytic converter oxygen sensor. This sensor monitors the oxygen content in the exhaust gases after they have been treated by the catalytic converter. A properly functioning downstream sensor in a good system typically produces a relatively stable, high voltage signal (often between 0.6V and 0.8V), indicating low oxygen content and efficient catalyst operation. Code P0138 triggers when the ECM sees this signal stuck at or repeatedly peaking at an unusually high voltage threshold (typically near 1.0V), which it interprets as a fault in the sensor circuit. This fault prevents accurate monitoring of the catalytic converter. The first diagnostic step is to connect a professional obd2 scanner to confirm the code and examine the sensor’s live data.

Symptoms of Error Code P0138

• Illuminated Check Engine Light (CEL).
• You may also see a catalytic converter efficiency code (e.g., P0420), as the falsely high signal can mislead the ECM about the catalyst’s performance.
• The vehicle will fail an emissions test.
• Emissions readiness monitors will be incomplete.
• Typically, there are no drivability symptoms such as rough idle, hesitation, or power loss. The downstream sensor does not control the air/fuel mixture.

Main Causes of Error Code P0138

• Faulty Downstream Oxygen Sensor (Bank 1, Sensor 2): The most common cause. The sensor’s sensing element has failed, causing it to output a continuously high signal voltage.
• Short to Power in the O2 Sensor Signal Wire: The wire carrying the sensor signal back to the ECM is frayed and touching a 12V or 5V power source (e.g., a nearby harness), forcing the signal voltage high.
• Poor Ground Connection for the Sensor: A bad or corroded ground wire for the O2 sensor can cause the signal voltage to float high.
• Problem with the ECM Reference Voltage: An internal issue in the ECM providing an incorrect reference voltage to the sensor circuit.
• Contaminated Oxygen Sensor: In some cases, certain types of contamination (less common than with upstream sensors) can skew the reading high.
• Engine Running Excessively Rich (less likely to cause P0138 directly): While a severe rich condition can cause a high sensor reading, the downstream sensor is buffered by the catalytic converter. A true rich condition would likely trigger upstream sensor (P0132) and fuel trim codes first.

How to Diagnose Error Code P0138?

1. Initial Scan and Live Data Review: Use a professional obd2 diagnostic scanner to read code P0138 and any related codes. Access live data and monitor the voltage for “Bank 1, Sensor 2.” A key observation: If the voltage is stuck at or very near 1.0V (or the vehicle’s reference voltage) and does not fluctuate, it strongly suggests a hard circuit fault—a short to power. If it is high but varies slightly, it may indicate a faulty sensor.
2. Visual Inspection of Sensor and Wiring: Safely raise the vehicle. Locate the downstream O2 sensor. Carefully inspect its wiring pigtail, especially where it routes near other harnesses, the fuel lines, or the chassis. Look for melted insulation, chafing, or any signs the signal wire could be contacting a power wire.
3. Test the O2 Sensor Circuit for Short to Power: This is the most critical test for P0138.
   • Disconnect the electrical connector from the O2 sensor.
   • Turn the ignition ON (engine off).
   • Using a digital multimeter, check the voltage on the signal wire terminal in the vehicle’s harness connector.
   • Expected Result: The voltage should be very low (less than 0.1V).
   • Fault Indicated: If you read battery voltage (~12V) or a steady 5V reference voltage, the signal wire is shorted to a power source somewhere between the sensor connector and the ECM.
4. Test the Oxygen Sensor Itself: If the circuit test shows no short to power, the sensor may be faulty. With the engine fully warmed up, use your vehicle diagnostics tool to graph the sensor voltage. Create a temporary lean condition by inducing a small vacuum leak. A functioning sensor should eventually show a decrease in voltage. No response indicates a faulty sensor stuck high.
5. Check Sensor Ground: With the sensor disconnected and the ignition OFF, use a multimeter to check for continuity between the sensor ground wire pin (in the vehicle harness) and a clean chassis ground. There should be very low resistance. High resistance indicates a poor ground.

Possible Causes and Diagnostic Methods

Tools needed: A professional obd2 diagnostic tool with live data, a digital multimeter, jack and jack stands, basic hand tools, and vehicle service information for wiring diagrams.

How do I fix error code P0138? (Solutions to the Problem)

Simple Fixes

• Repair a Short to Power in Wiring: If testing confirms a short, you must locate the exact point where the signal wire is contacting a power wire. Repair the damaged insulation on both wires with solder and heat-shrink tubing. Re-route and secure the harnesses to prevent recurrence.
• Clean and Repair Ground Connection: Locate, clean, and secure the ground connection for the downstream O2 sensor circuit.

In-depth Diagnosis and Repair Solutions

• Replace the Downstream Oxygen Sensor (Bank 1, Sensor 2): This is the solution if the sensor is faulty and a short to power is ruled out. After installation, clear all codes with your car code reader and perform a drive cycle.
• Repair Damaged Vehicle Wiring Harness: For extensive damage, a section of the main harness may need repair or replacement.
• Address ECM Issues (Rare): If all wiring and the sensor are confirmed good, and a faulty reference voltage is suspected, professional diagnosis of the ECM may be required.

Fix faults based on symptoms

Common Error Code P0138 in Vehicles

P0138 is commonly seen across all vehicle makes. Due to the under-vehicle location of the downstream sensor, wiring issues are frequent.

• Trucks and SUVs (Ford, GM, Ram): The long frame rails and complex underbody harnesses increase the chance of wires chafing and shorting together.
• Older vehicles: Aging wire insulation becomes brittle and cracks, increasing the risk of shorts.
• Any vehicle after underbody work: If recent repairs were done (exhaust, fuel lines, etc.), the O2 sensor wiring may have been pinched or rerouted incorrectly, leading to a short.

P0138 Frequently Asked Questions

Is it safe to drive with a P0138 code?

Yes, it is generally safe from a drivability and mechanical safety perspective. The downstream sensor does not affect engine operation. However, your emissions system is not being monitored correctly, and you will fail an emissions test. It should be repaired, but it is not an emergency stop-driving issue.

What is the difference between P0137 and P0138?

P0137 indicates a “Low Voltage” condition in the downstream O2 sensor circuit. P0138 indicates a “High Voltage” condition. P0138 is more specifically associated with a short to power in the signal wire, while P0137 is more often linked to a short to ground or an exhaust leak.

Will a P0138 code damage my catalytic converter?

The code itself will not damage the catalytic converter. However, if the root cause is a genuine, severe rich-running condition that is also causing the high sensor reading (less common), then that rich condition could overheat and damage the converter over time. The P0138 code is more often a circuit electrical fault.

How much does it cost to fix P0138?

Costs vary: Diagnosis: $80-$150. A new downstream O2 sensor: $80-$250. Labor: 0.5-1.5 hours ($50-$150). Repairing a wiring short can range from $100 to $300 depending on complexity. Total typically ranges from $150 to $500.

Do I need a special scan tool to diagnose P0138?

A basic obd reader can confirm the code. However, a professional obd2 diagnostic scanner that displays live sensor voltage is essential to see the stuck high reading. A digital multimeter is absolutely critical for performing the short-to-power circuit test.

Will clearing the code with a scanner make P0138 go away?

It will temporarily turn off the light. However, if the short circuit or faulty sensor is still present, the code will return as soon as the ECM completes its self-test on the next drive cycle. The underlying problem must be fixed.

P0138 Related OBD2 Errors

• P0136: O2 Sensor Circuit (Bank 1, Sensor 2)
• P0137: O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)
• P0140: O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 2)
• P0141: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)
• P0158: O2 Sensor Circuit High Voltage (Bank 2, Sensor 2)
• P0420: Catalyst System Efficiency Below Threshold (Bank 1) – A frequent companion code.

Important: Code P0138 is a clear indicator of a high voltage fault in the downstream oxygen sensor circuit. Diagnosis should immediately focus on checking for a short to power in the signal wire using a multimeter. Do not automatically replace the oxygen sensor. If a short is found and repaired, the original sensor may still be perfectly functional. Always verify the repair by clearing the code with your vehicle code reader and confirming it does not return after a test drive.