OBD-II Code P0138 — What It Means & How to Fix It

What Does This Error Mean?

P0138 means the downstream oxygen sensor on Bank 1 is reading a voltage that's too high. This is the O2 sensor located after the catalytic converter (Sensor 2). A high voltage reading tells the computer that the exhaust is too rich (too much fuel). The sensor monitors catalytic converter efficiency and helps fine-tune emissions. Common causes include a failing sensor, a rich running condition, or a bad catalytic converter.

Affected Models

All vehicles 1996+
Common in Toyota Camry
Common in Honda Accord
Common in Chevrolet Silverado
Common in Ford Escape

Common Causes

Faulty downstream O2 sensor stuck reading high voltage
Engine running too rich due to leaking fuel injectors or a faulty fuel pressure regulator
Failing catalytic converter not processing exhaust gases properly
Short circuit in the O2 sensor signal wire causing a false high reading
Coolant or oil contaminating the O2 sensor element from internal engine leaks

How to Fix It

Check for other stored codes first. If you also have misfire codes or fuel system codes, fix those first. A rich-running engine will cause the downstream sensor to read high voltage.

P0138 can be a symptom of other problems. Fixing the root cause may clear this code automatically.
Locate the Bank 1, Sensor 2 oxygen sensor. It's threaded into the exhaust pipe after the catalytic converter. Follow the exhaust pipe from the engine past the cat to find it.

Don't confuse it with Sensor 1, which is before the catalytic converter. Sensor 2 is always downstream.
Inspect the sensor's wiring and connector for damage. Look for melted insulation, corrosion, or wires touching bare metal. A short to voltage in the signal wire causes false high readings.

The wiring for this sensor runs along the underside of the car where it's exposed to road debris and moisture.
Replace the downstream O2 sensor if no other codes are present and the wiring looks fine. Use an O2 sensor socket and apply anti-seize to the new sensor's threads.

Downstream O2 sensors cost $25-$80. They're usually easier to access than the upstream sensor.
Clear the code and drive for 20-30 minutes. Monitor with a scan tool to see if the downstream sensor voltage stays between 0.1 and 0.9 volts and switches normally.

A healthy downstream sensor should hold a fairly steady voltage, unlike the upstream sensor which switches rapidly.

When to Call a Professional

If replacing the sensor doesn't fix it, the catalytic converter may be failing. A rich-running engine needs a mechanic to check fuel system components. A mechanic can test the catalytic converter's efficiency with a scan tool. Diagnosis costs about $80-$150. Catalytic converter replacement is expensive, ranging from $500 to $2,000.

Frequently Asked Questions

What does 'high voltage' mean for an O2 sensor?

O2 sensors produce a voltage between 0 and 1 volt. High voltage (above 0.9 volts) means the exhaust has very little oxygen. This indicates a rich condition with too much fuel in the mixture. The computer sets P0138 when this voltage stays high for too long. A properly working sensor should fluctuate between low and high readings.

Can P0138 be caused by a bad catalytic converter?

Yes, a failing catalytic converter can trigger P0138. When the converter stops cleaning exhaust gases effectively, it changes the O2 levels. The downstream sensor picks up this abnormal reading. If you also have a P0420 code, the converter is likely the problem. Replace the converter only after confirming the sensor itself is working correctly.

What is the difference between Sensor 1 and Sensor 2?

Sensor 1 is the upstream sensor, before the catalytic converter. It controls the engine's fuel mixture in real time. Sensor 2 is the downstream sensor, after the catalytic converter. Its main job is to verify the catalytic converter is working properly. Sensor 2 doesn't directly control fuel delivery like Sensor 1 does.