Common Causes of a P0138 Code: O2 Sensor High Voltage

P0138 Code: Causes, Symptoms, and Fixes Explained

Quick Summary
When I diagnose a P0138 code, I know the engine computer is detecting a high-voltage signal from the rear oxygen sensor (Bank 1, Sensor 2). That usually means the sensor is reporting a consistently rich condition—or the circuit itself is faulty. In most cases, the root cause is a
• failing O2 sensor
• wiring issue
• exhaust leak,
• or a true rich-running condition.
The fix comes down to verifying sensor data, checking wiring integrity, and confirming whether the engine is actually running rich or the sensor is lying.

What the P0138 Code Really Means

The P0138 code is defined as: “O2 Sensor Circuit High Voltage (Bank 1, Sensor 2).”

Your downstream oxygen sensor—mounted after the catalytic converter—should produce a relatively stable voltage, usually around 0.6–0.8 volts. When the PCM sees that the voltage is stuck high (typically above 0.9V) for too long, it sets the P0138 code.

This sensor isn’t primarily used for fuel control—that’s the upstream sensor’s job. Instead, it monitors catalytic converter efficiency. So when I see a P0138 code, I’m thinking: is the engine actually running rich, or is the sensor circuit lying?

Symptoms of a P0138 Code

In many cases, a P0138 code won’t cause major drivability problems—but there are still clues you shouldn’t ignore:

• Check engine light illuminated
• Possible decrease in fuel economy
• Strong fuel smell from the exhaust
• Failed emissions test
• Occasional rough running (if a true rich condition exists)

In my experience, many drivers don’t notice anything beyond the check engine light. That’s why proper diagnosis matters.

Most Common Causes of a P0138 Code

After years of diagnosing oxygen sensor issues, I can tell you these are the most frequent causes of a P0138 code:

1. Faulty Rear Oxygen Sensor (Most Common) — The sensor itself can fail internally and get stuck at a high voltage reading.
2. Short to Voltage in the Signal Circuit — A wiring issue can feed constant voltage to the PCM, mimicking a rich condition.
3. Engine Running Rich — A really rich condition can push O2 voltage high. Causes include:
• Leaking fuel injector
• High fuel pressure
• Faulty mass airflow sensor
4. Exhaust Leak Near the Sensor — While less common, certain exhaust issues can skew readings.
5. Contaminated Sensor — Oil, coolant, or fuel contamination can damage the sensor and cause false readings.
If you want to cut to the chase, a P0138 is usually caused by:
• Bad downstream O2 sensor or wiring harness issue
• Vacuum leak or exhaust leak.
• Leaking fuel injectors or high fuel pressure.
• Bad catalytic converter (lost its ability to store oxygen)

How I Diagnose a P0138 Code

When I’m dealing with a P0138 code, I don’t just replace the sensor—I verify the problem first.

Step 1: Check Live Data — I look at the downstream O2 sensor voltage:
• If it’s stuck above 0.9V, that confirms the condition
• I compare it to upstream sensor activity
• If the upstream is switching normally and the downstream is stuck high, I suspect the sensor or circuit.
Step 2: Force a Lean Condition — This is one of my go-to tests. I introduce a controlled vacuum leak:
• The O2 sensor voltage should drop quickly
• If it doesn’t, the sensor is likely faulty
Step 3: Inspect Wiring and Connectors — I check for:
• Melted wiring near the exhaust
• Corrosion at connectors
• Shorts to power
• Wiring problems are more common than most people think.
Step 4: Check for a True Rich Condition — I review fuel trim data:
• Negative fuel trims indicate a rich condition
• I inspect injectors, fuel pressure, and airflow data
Step 5: Check for Exhaust or Contamination Issues — If everything else checks out, I inspect:
• Exhaust leaks
• Oil or coolant contamination
• How to Fix a P0138 Code
• Once you’ve identified the cause, fixing a P0138 code is usually straightforward.
Most Common Fix:
• Replace the rear oxygen sensor
• Other Possible Repairs:
• Repair or replace damaged wiring
• Fix short-to-voltage issues
• Replace leaking fuel injector
• Correct high fuel pressure condition
• Repair exhaust leaks
In my experience, about 70–80% of P0138 cases are resolved with a new downstream O2 sensor—but only after confirming the circuit is good.

Why the P0138 Code Shouldn’t Be Ignored

Even though the P0138 code may seem minor, ignoring it can lead to:

• Reduced fuel efficiency
• Increased emissions
• Potential catalytic converter damage over time
• Failed emissions testing

Fixing it early is quick, inexpensive, and prevents bigger issues.

Pro Tips From the Shop

Here are a few insights I’ve learned diagnosing the P0138 code:
• Don’t replace the sensor without checking wiring first
• Always compare upstream vs downstream O2 behavior
• Use live data—not just stored codes
• A stuck high voltage doesn’t always mean a rich engine
• Cheap aftermarket sensors can cause repeat failures

©, 2024 Rick Muscoplat