P0155: Bank 2 Sensor 1 O2 Heater Circuit Issue
P0155 Code: O2 Sensor Heater Circuit Malfunction. See symptoms, common causes, driving risk and checks that help confirm the fault before parts are replaced.
Save all codes and freeze-frame data. The code tells you what the control module detected, not which component must be replaced.
Quick answer
Code P0155 indicates a malfunction in the heater circuit of the upstream oxygen (O2) sensor on the engine's Bank 2, Sensor 1 position. This means the ECM has detected an issue with the sensor's internal heating element, which is designed to bring the sensor to operating temperature quickly for accurate exhaust gas readings. The code does not mean the sensor itself is bad, but that its heater circuit is failing to function as expected.
How the P0155 Code Relates to the O2 Sensor Heater Circuit (Bank 2 Sensor 1)
The P0155 code is triggered when the Engine Control Module (ECM) detects a fault in the heater circuit of the Bank 2, Sensor 1 O2 sensor. Bank 2 refers to the side of the engine that does not contain cylinder #1 (in a V-engine layout), while Sensor 1 denotes the upstream or pre-catalytic converter sensor. This sensor is critical for measuring oxygen levels in the exhaust to help the ECM adjust the air-fuel mixture for optimal combustion and emissions control.
The heater circuit inside the O2 sensor is a small resistive element that warms the sensor to ~600°F (315°C) within seconds of a cold start. Without this heater, the sensor would take several minutes to reach operating temperature, leading to poor fuel economy and increased emissions during warm-up. The ECM monitors the heater circuit by checking for proper resistance and voltage drop. If it detects an open circuit (no continuity), short to ground, or excessive resistance (often due to a broken wire or damaged heater element), it logs P0155.
Common failure modes include a blown fuse (if the heater circuit is fused separately), a broken or corroded wiring harness near the sensor connector, or an internal open in the heater element itself. In some vehicles, the ECM also monitors the heater's current draw; if it's too high or too low, P0155 may set. Unlike a standard O2 sensor code (e.g., P0131 for a slow-response sensor), P0155 specifically points to the heater portion of the circuit, not the sensor's ability to read oxygen levels once warm.
It's worth noting that modern O2 sensors often combine the sensing and heating elements into a single unit. If the heater fails, the sensor may still provide intermittent readings once the exhaust gases heat it naturally, but cold-start performance and emissions will suffer. The ECM may also set additional codes if the lack of heater functionality leads to prolonged lean or rich conditions.
Common Symptoms of P0155
Drivers often notice no immediate drivability issues, as the engine can still run on input from other sensors. However, common symptoms include a illuminated check engine light, slightly reduced fuel economy (especially in cold weather), and a rough idle or hesitation during the first few minutes of operation. In some cases, the vehicle may fail an emissions test due to elevated hydrocarbons or carbon monoxide during the warm-up phase.
Typical Causes and Diagnosis
P0155 is most frequently caused by a faulty O2 sensor heater element, damaged or corroded wiring in the sensor's circuit, or a blown fuse (where applicable). Less commonly, a poor connection at the sensor or ECM, or internal ECM failure, may be to blame. Diagnosis typically involves checking the heater circuit resistance with a multimeter, inspecting wiring for breaks or corrosion, and verifying power and ground at the sensor connector. A scan tool can also monitor the sensor's heater voltage or current draw in real-time.
Can You Still Drive With Code P0155?
Yes, you can usually continue driving with P0155, as it does not pose an immediate safety risk or cause severe drivability issues. However, prolonged driving with this code may lead to reduced fuel efficiency, increased emissions, and potential damage to the catalytic converter over time if the air-fuel mixture runs excessively rich or lean. It's best to address the issue promptly to avoid secondary problems.
Related Codes
P0135 ; often appears alongside P0155 as it's the equivalent code for Bank 1, Sensor 1, indicating a similar heater circuit issue on the opposite side of the engine. P0141 ; may accompany P0155 if the downstream O2 sensor (Bank 2, Sensor 2) heater circuit is also faulty, though this is less common.
Details that improve the diagnosis
Compare the reported value with engine temperature, barometric pressure, throttle position and related sensors. A signal that is electrically valid can still be implausible. Fuel trims at idle and at a steady higher speed help separate an intake leak from a fuel-delivery or measurement problem.
Before clearing anything, record whether the code is current, pending, history or permanent. Save companion codes and freeze-frame conditions, then note recent repairs, a weak battery, weather changes and the exact moment the symptom appears. After a confirmed repair, clear only the erasable codes and complete the appropriate drive cycle. A permanent code normally clears only after the onboard monitor verifies that the fault is gone.