P0155: O2 Sensor Heater Circuit Failure (Bank 2 Sensor 1)

What is P0155?

The diagnostic trouble code P0155 is defined as “O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1).” 

The oxygen sensor heater circuit (Bank 2 Sensor 1) is located in the exhaust manifold before the catalytic converter. This heated oxygen sensor (also called an O2 sensor or air-fuel sensor) monitors the oxygen levels in the exhaust before it enters the catalytic converter. 

The engine control module (ECM) uses information from the upstream oxygen sensor to adjust the air-fuel ratio to an optimum level (around 14.7:1).

To provide accurate data quickly, the air-fuel sensor (AF sensor) uses built-in heater elements to reach operating temperature (600°F or 316°C) faster. The ECM logs code P0155 when the heater element within the oxygen sensor heater circuit takes longer than usual to heat up. 

NOTE: “Bank 2” is the engine side with cylinder 2 (the driver or passenger side, depending on the layout). Meanwhile, “Sensor 1” refers to the upstream oxygen sensor.

What Are the Common Symptoms of a P0155 Code?

Here are some early code P0155 signs to keep an eye out for:  

1. Flashing check engine light: This can happen if the O2 sensor’s heating element fails to reach the right operating temperature. Alternatively, a faulty coolant sensor that sends incorrect signals and disrupts fuel mixture adjustments can also trigger the dashboard light.

2. Weird engine noises: A faulty sensor heater can throw off the air-fuel ratio, causing the engine to run lean or rich. This imbalance can lead to rough combustion or engine misfires, which may produce strange sounds.

3. Presence of black smoke or a rotten egg-like smell: Running rich can cause unburnt fuel to enter the exhaust, leading to black smoke or a sulfur-like, rotten egg odor from the tailpipe.

4. Rough idling: A faulty sensor heater can disrupt the fuel mix at startup, causing the engine to shake or idle unevenly. Damaged sensor wires or connectors may also be to blame.

5. Spike in fuel consumption: A faulty O2 sensor heater circuit can misread oxygen levels, prompting the ECM to send excess fuel to the engine. This leads to higher fuel consumption and possibly a P0155 trouble code.

6. Failed emissions test: If the O2 sensor doesn’t heat up quickly, the engine may burn excess fuel and emit more pollutants, often causing emissions test failure.

What Are the Common Causes of a P0155 Code?

Depending on your car’s make and model, several issues could trigger a P0155 trouble code:

• A faulty pre-catalyst oxygen sensor
• A malfunctioning O2 heater circuit 
• An open circuit, short to voltage, short to ground, or short circuit in the heater wire
• A blown fuse in the O2 sensor heater circuit
• The O2 heater element is faulty 
• The O2 sensor heating element has high resistance
• A damaged O2 sensor connector
• Broken or bare wires leading to the AF sensor
• A faulty spark plug 
• A malfunctioning engine coolant temperature sensor
• Low car battery voltage
• The hose to the exhaust gas recirculation (EGR) system module is disconnected
• Corroded or loose electrical terminals at the O2 sensor connector or the PCM (Powertrain Control Module) connector
• Failing PCM 

Can You Still Drive with a P0155 Code?

Ideally, you should avoid driving and rectify the issue ASAP.

This is because the P0155 diagnostic trouble code indicates an imbalance in the fuel-to-oxygen ratio. This imbalanced mixture can cause high fuel consumption, poor engine performance, drops in power, and sluggish acceleration.

If left undiagnosed, it can lead to severe engine damage.

How to Diagnose a P0155 Code: A General Know-How

Diagnosing a P0155 code requires a deep understanding of the O2 sensor heater circuit. If you’re unsure, it’s best to let a trusted mechanic diagnose the problem.

Here’s how they get some answers:

1. The mechanic will use an OBD-II scanner to retrieve the P0155 code from the ECM.

2. They’ll inspect the O2 sensor heater circuit wiring and connectors for damage and signs of poor maintenance. This includes frayed wires, poor contact, corrosion, or loose terminals at the sensor or PCM.

3. They’ll check the fuse powering the O2 sensor heater circuit to see if it’s blown.

4. Using a multimeter, they’ll test the heater circuit for continuity and check for shorts to voltage or ground.

5. They’ll check the heater element to see if it has low or high resistance, or if the component itself is faulty.

6. Using the ODB-II scanner, they’ll review live data from the ECM to assess the O2 sensor heater’s performance.

7. They’ll also check live data from the engine coolant temperature sensor to ensure it’s working correctly.

8. Using a multimeter or battery tester, they’ll test the car battery and charging system to rule out low voltage.

9. If needed, they’ll inspect the spark plugs for faults.

10. They’ll confirm the hose to the EGR system module is connected properly.

11. If everything else checks out, they’ll test the ECM/PCM using a bidirectional OBD-II scanner or multimeter to see if it’s sending the correct signals or has internal faults.

Possible Repairs and Costs for a P0155 Code

Depending on the root problem, here are the potential fixes for a P0155 error code, along with how much money they may cost (with labor):

• Blown fuse replacement: $75-$130
• Wiring repair: $50 to $200
• Spark plug replacement: $50 to $750
• EGR hose replacement: $250 to $350
• O2 sensor replacement: $360 to $520
• Coolant temperature sensor replacement: $400 to $500
• Battery replacement: $185 to $500+ (standard), $4,000 to $20,000 (EV)
• ECM replacement: $800 to $1,500

How Long Does It Take to Fix a P0155 Code?

The answer to this question depends on the underlying issue triggering the P0155 diagnostic trouble code.

Regular maintenance or simple repairs, such as replacing a blown fuse or reconnecting a loose heater wire, can take 30 minutes to an hour. 

However, if you need a new sensor or the wiring harness is damaged, it could take 1 to 2 hours. ECM-related repairs may take longer due to diagnostics and reprogramming.