O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
The P0155 code indicates a malfunction in the heater circuit of the oxygen sensor located before the catalytic converter on Bank 2 (the side of the engine opposite cylinder #1). The heater element is essential for the sensor to reach operating temperature quickly, especially during cold starts. When the PCM detects that the heater circuit is drawing too much or too little current, or that the sensor is not warming up as expected, it sets this code. This can lead to inaccurate air-fuel ratio readings, reduced fuel economy, and increased emissions.
1. Most Common Causes
- Faulty oxygen sensor (Bank 2, Sensor 1) with an open or high-resistance heater element
- Blown fuse in the O2 sensor heater power supply circuit
- Damaged wiring or connectors (short to ground, short to voltage, open circuit, corrosion) in the heater circuit
- Defective O2 heater relay (if applicable)
- Faulty engine control module (PCM/ECM) – rare
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Poor fuel economy
- Failed emissions test
- Rough idle or hesitation
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm P0155 and check for other codes.
Step 2: Visually inspect the O2 sensor connector and wiring for damage, corrosion, or loose connections.
Step 3: Check the heater circuit fuse (usually labeled O2 or HTR) and replace if blown.
Step 4: Measure resistance across the heater element pins of the sensor; typical range is 2-30 ohms. If out of spec, replace the sensor.
Step 5: Test for battery voltage and ground at the sensor connector with the key on. If missing, repair wiring or PCM issue.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, but it may cause poor fuel economy and increased emissions. Prolonged driving could damage the catalytic converter.
How much does it cost to fix?▼
Typically $50-$150 for a DIY replacement of the oxygen sensor. Labor adds $50-$100 if done by a shop.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.