P0138 OBD2 Code Guide
P0138 is a sensors diagnostic trouble code. Use this hub to understand the plain meaning, the first checks to run, and which vehicle-specific guides may be more useful before replacing parts.
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
Code P0138 indicates that the Powertrain Control Module (PCM) has detected a persistently high voltage signal from the downstream oxygen sensor (Sensor 2) on Bank 1. This sensor is located after the catalytic converter and is primarily responsible for monitoring the converter's efficiency by measuring the oxygen content in the exhaust gases. A consistently high voltage (typically above 0.9 volts) suggests that the exhaust gas is very rich in fuel, or there is an electrical fault in the sensor circuit itself, or the sensor is contaminated/failing. This can lead to increased emissions, reduced fuel economy, and potential long-term damage to the catalytic converter.
Code verification standard
P0138 is published with SAE J2012 / ISO 15031-6 code structure plus OBD2 P0138 diagnostic context.
First checks
- - Read all stored and pending codes before clearing anything.
- - Save freeze-frame data so you know speed, load and temperature when the code set.
- - Inspect wiring, connectors, fuses, hoses and obvious leaks tied to the named system.
- - Compare live data against realistic values before buying sensors or modules.
- - Clear the code only after repair and complete a drive cycle to confirm it does not return.
P0138 diagnostic path
- 1**Visual Inspection:** Carefully inspect the Bank 1 Sensor 2 O2 sensor, its wiring harness, and connector for any signs of damage, corrosion, fraying, or loose connections. Also, check for exhaust leaks near the sensor or upstream of it, which can introduce ambient air and skew readings.
- 2**Scan Tool Live Data Analysis:** Connect an OBD2 scan tool and monitor the live data for Bank 1 Sensor 2 voltage. Observe if the voltage is indeed stuck high (e.g., consistently above 0.9V) and if it responds to throttle input. Compare its readings to Bank 2 Sensor 2 (if applicable) or to specifications.
- 3**Check Sensor Heater Circuit:** Disconnect the O2 sensor and use a multimeter to check the resistance of the heater circuit terminals on the sensor itself. Compare the reading to manufacturer specifications. Also, check for continuity and proper voltage supply to the heater circuit at the harness connector.
- 4**Verify Fuel Trim and Engine Condition:** Use the scan tool to check short-term and long-term fuel trims for Bank 1. Excessively negative fuel trims could indicate a rich running condition. Perform a fuel pressure test and inspect fuel injectors for leaks to rule out actual rich conditions.
- 5**Sensor Response Test:** If the sensor appears to be stuck high, try introducing a controlled vacuum leak or propane enrichment (carefully!) to see if the sensor voltage responds. If it doesn't respond or responds very slowly, the sensor is likely faulty.
Verify before repair
- - **Replace the Bank 1 Sensor 2 O2 Sensor:** This is the most common fix, as the sensor itself often fails, becoming slow to respond or stuck in a high voltage state due to contamination or age.
- - **Repair or Replace Damaged Wiring/Connector:** Address any issues with the O2 sensor's electrical circuit, including frayed wires, corroded terminals, or loose connections.
- - **Repair Exhaust Leaks:** Locate and seal any exhaust leaks, especially those upstream of the O2 sensor, which can cause false rich readings.
- - **Diagnose and Repair Rich Running Condition:** If the sensor is accurately reporting a rich condition, diagnose the root cause such as a leaking fuel injector, excessive fuel pressure, faulty EVAP purge valve, or a contaminated Mass Air Flow (MAF) sensor.
Vehicle-specific pages to check next
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