P0155
O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
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The P0155 code indicates a problem with the heater circuit in the upstream oxygen sensor on Bank 2. Every modern heated O2 sensor contains a small electric heater that rapidly warms the sensor's ceramic element to its operating temperature of around 600°F. This allows the sensor to begin providing accurate air-fuel ratio feedback within seconds of starting the engine rather than waiting for exhaust heat to do the job.
When the heater circuit fails, the sensor must rely solely on exhaust gas heat to reach operating temperature, which can take several minutes. During this extended warm-up period, the PCM operates in open-loop mode using default fuel maps rather than real-time sensor feedback. You may notice slightly rough running or reduced fuel economy during the first few minutes after a cold start, but once the engine warms up, symptoms typically disappear.
The most common fix is replacing the O2 sensor since the heater element is built in and not separately serviceable. However, always check the heater circuit fuse and relay first — this is the simplest and cheapest potential fix. Also inspect the wiring to the sensor for heat damage or corrosion, especially near the exhaust manifold where temperatures are highest. This is a beginner-friendly DIY repair on most vehicles, typically requiring only an O2 sensor socket and a basic ratchet set.
Severity
Symptoms
- •Check engine light is on
- •Slightly reduced fuel economy during short trips
- •Engine may run rough briefly after cold start
- •Vehicle fails emissions testing
- •Longer warm-up period before smooth idle
- •No symptoms once engine is fully warmed up
Likely Causes
The internal heating element in the upstream O2 sensor on Bank 2 has burned out or developed an internal open circuit, preventing it from heating the sensor to operating temperature quickly.
Damaged, corroded, or melted wiring in the heater power or ground circuit prevents current from reaching the heater element.
A blown fuse or faulty relay in the O2 heater circuit cuts power supply to the sensor's heater element.
The transistor or driver within the PCM that controls heater ground or power has failed, though this is an uncommon cause.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the upstream O2 sensor (Bank 2, Sensor 1) — the heater is integrated into the sensor
- Check and replace the O2 sensor heater fuse if blown
- Repair damaged or corroded wiring in the heater circuit
- Verify proper power and ground at the sensor connector
- Clear codes and verify repair with a test drive
Shop Parts
Videos
P0155 Explained - Oxygen Sensor Heater Circuit Malfunction (Simple Fix)
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Causes and Fixes P0155 Code: Oxygen Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
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