Quick answer: The P0155 code indicates a malfunction in the heater circuit of the upstream oxygen sensor on Bank 2. It affects the engine’s emissions and fuel control system. While it is not an immediate safety emergency, you should diagnose and repair it promptly to prevent catalytic converter damage and poor fuel economy.
Seeing the check engine light illuminate on your dashboard can be stressful, especially when a scanner reveals code P0155. This code points specifically to a hiccup in how your engine measures oxygen in the exhaust during cold starts. In this guide, we will break down what this code means, how to spot the symptoms, and the steps you can take to resolve it without overspending.
What Does P0155 Mean?
P0155 stands for “Oxygen Sensor Heater Circuit Malfunction (Bank 2, Sensor 1).” To reduce emissions, your car’s engine control module (ECM) needs the oxygen sensors to reach their operating temperature of about 600°F as quickly as possible. To speed this up, these sensors have internal heating elements. Bank 2 refers to the side of the engine that does not contain cylinder number one, and Sensor 1 is the upstream sensor located before the catalytic converter. When the ECM detects that the heater circuit for this specific sensor is taking too long to warm up or has an electrical fault, it triggers the P0155 code.
Symptoms
- Active check engine light on the dashboard.
- Increased fuel consumption, as the engine stays in “open-loop” mode longer.
- Rough idling or slight hesitation during cold starts.
- A stronger smell of fuel from the exhaust pipe.
- Failed emissions test due to incomplete monitor readiness.
Possible Causes
Before buying any parts, keep in mind that the most common mistake is immediately replacing the oxygen sensor without checking the wiring first.
- Damaged, burnt, or corroded wiring harness leading to the sensor.
- Blown fuse or faulty relay powering the oxygen sensor heater circuit.
- Failed heating element inside the Bank 2, Sensor 1 oxygen sensor.
- Loose or corroded electrical connectors.
- Engine control module (ECM) software issues or internal failure.
Check These Yourself Before Visiting a Mechanic
- Inspect the wiring harness near the Bank 2 Sensor 1 (upstream sensor on the non-cylinder-1 side) for any signs of melting from exhaust heat or damage from road debris.
- Check your vehicle’s fuse box for any blown fuses related to the oxygen sensors or auxiliary heater circuits.
- Disconnect the sensor plug, inspect it for corrosion or bent pins, and firmly reconnect it to ensure a solid electrical connection.
How to Fix It
- Perform a visual inspection of the wiring, connectors, and fuses, replacing any blown fuses or repairing damaged wires.
- Use a digital multimeter to test for 12-volt power and ground at the sensor’s harness connector with the key in the on position.
- Measure the resistance of the heater circuit directly on the oxygen sensor pins; if it shows “open” or infinite resistance, the sensor’s heater is broken.
- Replace the faulty Bank 2, Sensor 1 oxygen sensor if the wiring tests fine but the sensor fails the resistance test.
- Clear the trouble code using an OBD-II scanner and perform a test drive to verify the repair.
Estimated Repair Costs (2026)
| Job | Parts ($) | Labor ($) |
|---|---|---|
| Diagnostic Fee | $0 | $50 – $150 |
| Fuse or Wiring Repair | $5 – $20 | $50 – $100 |
| O2 Sensor Replacement | $50 – $150 | $70 – $150 |
Note: these are 2026 estimates in US dollars; costs vary by make and model.
Frequently Asked Questions
Can I drive with the P0155 code?
Yes, you can safely drive the vehicle short-term. However, because the engine will run less efficiently during warm-up, driving long-term with this code can decrease your fuel economy and eventually cause premature wear on your catalytic converter.
How do I clear the P0155 code?
You can clear the code using an OBD-II scan tool plugged into your vehicle’s diagnostic port. If you have repaired the underlying issue, the check engine light will stay off; if the problem remains, the ECM will trigger the code again within one or two drive cycles.
Where is Bank 2 Sensor 1 located?
Bank 2 is the cylinder bank that does not contain cylinder number one. Sensor 1 is the upstream oxygen sensor, which is located in the exhaust manifold or pipe before the catalytic converter.