Safe to drive short-term. Repair within 1–2 weeks. P0138 means the post-catalytic converter heated oxygen sensor on Bank 1 (Sensor 2) is reading above 1.2 V for more than 10 seconds, indicating a shorted signal circuit, a failed sensor stuck rich, or genuine exhaust contamination keeping the sensor biased high.
What P0138 means
The heated oxygen sensor behind the catalytic converter (HO2 sensor, Bank 1 Sensor 2) generates output voltage between approximately 0.1 V and 0.9 V under normal operation, switching slowly around the stoichiometric point. P0138 is a high-voltage fault detected continuously: if the HO2 sensor voltage exceeds 1.2 V for 10 seconds or more, the ECM determines the output is abnormally elevated and stores the code after two consecutive failing drive cycles. A voltage above 1.2 V cannot be produced by normal exhaust oxygen variation — it points to either an electrical short in the signal circuit (typically a short to the sensor heater supply or battery voltage), a sensor internally shorted, or in rare cases sustained extreme richness in the exhaust allowing the electrochemical cell to produce an elevated voltage. The monitor activates 2 seconds after engine start and requires battery voltage above 11 V.
Symptoms
- Malfunction indicator lamp (check engine light) on after two failing driving cycles
- Failed emissions test — post-catalyst data shows an unrealistic rich bias
- Possible enrichment of fuel trims if the ECM uses downstream sensor feedback — can cause slightly rich running and elevated fuel consumption
- No noticeable drivability symptom in most cases
- Possible rotten-egg exhaust odour if the engine is genuinely running extremely rich
Common causes
- Signal wire short to voltage — short circuit between the HO2 sensor signal wire and the heater supply wire or battery power inside the harness or at a chafed point
- Failed HO2 sensor with internally shorted sensing element — sensor output locked high
- ECM-side wiring fault — damaged ECM connector pin or an internal ECM short on the signal input
- Sustained extremely rich running condition from a separate fault (large fuel injector leak, stuck-open injector, failed mass airflow sensor reading very high)
Severity & driving advice
Severity: low — Emissions are degraded and catalyst monitor may be blocked. A wiring short, if present, risks incorrect fuel delivery over time.
Can I drive? Safe to drive short-term. Repair within 1–2 weeks.
Diagnostic approach
- Read live sensor voltage in Data List before disconnecting anything — With the engine warm and running, check the Sensor 2 Bank 1 voltage. If it reads above 1.2 V at idle, confirm the reading is not from a scan tool communication glitch — turn ignition off, reconnect, and recheck. A genuine P0138 will show persistently elevated voltage on the live data screen.
- Disconnect the HO2 sensor connector and re-check voltage — With the sensor connector unplugged, use a voltmeter to measure voltage between the signal wire pin on the harness side and chassis ground. A reading above 1 V here confirms a short to voltage in the harness — the sensor itself is not at fault in this case. A reading near 0 V with the sensor disconnected suggests the short is inside the sensor body.
- Inspect the harness for chafing near the exhaust — The HO2 sensor wiring runs close to the exhaust pipe. Heat-induced chafing can cause the signal wire to contact the heater power supply wire (which carries battery voltage). Follow the harness from the sensor back 30–50 cm, looking for melted insulation, pinch marks at heat shields, or areas where two wires are fused together.
- Check the ECM connector pin for damage — Inspect the ECM-side signal pin for corrosion, spread contact blades, or signs of moisture intrusion. A damaged or corroded pin can create an abnormal resistance path that elevates the measured signal voltage. Clean with electrical contact cleaner if corrosion is present.
- Confirm no extremely rich running condition — Check long-term and short-term fuel trim values. Trims consistently below -10 % point to a genuine over-rich condition that is saturating the sensor. Address fuel trim faults (P0172, large injector issues) before condemning the sensor circuit.
- Replace the HO2 sensor if harness and ECM connector checks are normal — If the harness tests clean and voltage drops to a normal range after sensor replacement, the internal sensing element was shorted. After replacement, run the HO2 sensor confirmation driving pattern: warm engine to ECT above 75 °C, drive steadily for 20 seconds or more to allow the sensor monitor to complete and confirm status NORMAL.
Make & model notes
Toyota: Toyota downstream sensors on V6 applications (1GR-FE: 4Runner, FJ Cruiser, Tacoma) are located where the sensor wiring passes near the frame and can chafe if original heat-shield clips are missing. Inspect the full harness run before ordering a sensor.
Honda: Honda Civic and Accord models with higher mileage occasionally develop P0138 from oil contamination of the downstream sensor — burning oil from worn valve stem seals coats the sensing element and can bias voltage high. Check for blue smoke before replacing the sensor.
FAQ
What is the voltage threshold for P0138?
The factory detection threshold is 1.2 V or higher for 10 seconds or more during continuous monitoring. Normal HO2 sensor output ranges between about 0.1 V (lean) and 0.9 V (rich). Any sustained reading above 1.2 V is outside the sensor's normal electrochemical operating range and indicates a circuit or sensor fault.
Is P0138 the opposite of P0137?
Yes, in practical terms. P0137 indicates the sensor is stuck low (below 0.21 V during a commanded rich condition), pointing to an open circuit or lean bias. P0138 indicates the sensor is stuck high (above 1.2 V), pointing to a short to voltage or a failed sensor element shorted internally. Both affect Bank 1 Sensor 2.
Will P0138 cause other codes?
P0138 can indirectly trigger fuel trim codes (P0172 rich) if the ECM is using downstream sensor data to modify fuel injection. It can also prevent the catalyst efficiency monitor (P0420) from running, because the downstream sensor data it depends on is flagged as invalid.