A rear oxygen sensor that reads high all the time tells the engine computer the exhaust is rich after the catalytic converter. That might be true, or the sensor circuit might be faulty. A P0138 code means the bank 1 downstream oxygen sensor voltage stayed too high for too long.
This guide explains what drives the signal high, how to test it and which repairs clear the code.
P0138 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 1 | Fuel and air metering | The fault involves the sensors that measure air and fuel. |
| 38 | Specific fault | O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) |
What the P0138 Code Means
The SAE J2012 standard defines the P0138 code as O2 Sensor Circuit High Voltage (Bank 1 Sensor 2). Sensor 2 sits after the catalytic converter on the side of the engine with cylinder 1.
A healthy rear sensor usually holds a steady reading between about 0.5 and 0.8 volt. A zirconia sensor cannot make much more than about 1.0 volt by itself. When the powertrain control module (PCM) sees the signal stay above about 1.0 volt, or stuck high when it should drop, it sets the code.
Many PCMs test the rear sensor during deceleration fuel cutoff. With no fuel going in, the exhaust becomes very lean, and the sensor should drop below about 0.2 volt. A sensor that stays high during that test fails.
Symptoms of a High Rear O2 Sensor Reading
Drivability is often normal, but some engines adjust fuel based on this sensor. Watch for these signs of a P0138 code:
- Check engine light on
- Catalyst monitor that will not complete
- Failed emissions test
- Slight hesitation if the PCM leans out the mixture
- Fuel smell if the engine truly runs rich
What Causes the P0138 Code?
High readings come from wiring shorts, contamination or rich running. These are the usual causes:
- Signal wire shorted to the heater wire. Heat can melt insulation inside the sensor cable.
- Signal wire shorted to another voltage source. Damaged wiring under the vehicle can touch a power wire.
- Failed oxygen sensor. An internal fault can lock the output high.
- Contaminated sensor. Oil, coolant or fuel additives coat the tip.
- Rich running engine. A leaking injector or high fuel pressure can push the rear reading up.
- Water in the connector. Moisture can bridge the signal and heater pins.
How to Diagnose the P0138 Code
Gather a scan tool with live data, a multimeter and the wiring diagram. Drive the vehicle until the converter is hot.
Check the Deceleration Response
Graph the bank 1 sensor 2 voltage during a test drive. Accelerate to about 40 mph, then let off the gas completely and coast. During coasting, the rear sensor should drop below about 0.2 volt within a few seconds. A sensor that stays above 0.8 volt fails this test.
Check Fuel Trims
Look at bank 1 short-term and long-term fuel trims. Large negative numbers mean the PCM is removing fuel to correct a rich condition. If trims look normal, the sensor or wiring is more likely at fault.
Test for a Short to Voltage
Unplug the sensor and turn the key on. Measure the signal wire on the PCM side of the connector. It should read near 0.45 volt or lower. A reading of battery voltage means the wire is shorted to power. Also measure between the signal and heater pins on the sensor itself. Any continuity between them means an internal short.
Fixing the P0138 Code
Match the repair to the test results. Clear the code and drive until the monitors complete.
- Replace the oxygen sensor if it shows an internal short or fails the coasting test.
- Repair damaged wiring that touches a voltage source.
- Dry and clean the connector if water got inside, and add dielectric grease.
- Fix the rich condition, such as a leaking injector or a bad fuel pressure regulator.
- Fix the contamination source before you install a new sensor.
Repair Cost Breakdown
The repair for a P0138 code is often just a sensor. This table shows typical costs.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Downstream O2 sensor replacement | $30 to $120 | $120 to $350 |
| Wiring repair | $10 to $30 | $100 to $300 |
| Fuel pressure regulator | $30 to $120 | $150 to $400 |
| Fuel injector replacement (each) | $50 to $200 | $150 to $500 |
Is It Safe to Drive with a P0138 Code?
Usually, yes. The engine runs on the front sensor for most fuel control. If the PCM uses the false rich signal to trim fuel, it may lean the engine a bit, which can cause a mild stumble.
A real rich condition is a bigger concern. Unburned fuel can overheat the catalytic converter, so fix any rich running problem quickly.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0134 | O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1) |
| P0135 | O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1) |
| P0136 | O2 Sensor Circuit Malfunction (Bank 1 Sensor 2) |
| P0137 | O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) |
| P0139 | O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) |
Bringing the Rear O2 Sensor Back to Normal
A P0138 code means the bank 1 downstream oxygen sensor reads rich for too long. Internal shorts, wiring damage and true rich conditions cause most cases.
- Check the sensor’s drop during coasting.
- Look at fuel trims for a rich condition.
- Test the signal wire and sensor for shorts to voltage.
- Replace the sensor if it stays high or shows an internal short.
Frequently Asked Questions
Why Does the Rear Sensor Read Over 1 Volt?
A zirconia sensor cannot produce much more than 1 volt on its own. A higher reading almost always means the signal wire is touching a voltage source, often the heater circuit inside the sensor. Test the sensor for a short between the signal and heater pins.
Can a Bad Catalytic Converter Cause the P0138 Code?
Rarely. A worn converter usually makes the rear signal switch like the front, which sets an efficiency code such as P0420. A rear signal stuck high points more to the sensor, the wiring or a rich condition. Test those before replacing the converter.
Is It Normal for the Rear Sensor to Read Steady?
Yes. A working converter evens out oxygen levels, so the rear sensor reads fairly steady, often around 0.5 to 0.8 volt. It should still drop low during coasting and rise when you accelerate hard. Lack of any movement suggests a fault.
Do Fuel Additives Affect the Rear Sensor?
Some additives and fuel contaminants can coat oxygen sensors over time. Silicone and leaded fuel are the worst offenders. Use quality fuel and avoid products that are not marked safe for oxygen sensors and catalytic converters.