P0155: O2 Sensor Heater Circuit Malfunction (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 AF sensor) monitors the oxygen levels in the exhaust before it goes into the catalytic converter. Your vehicle’s engine control module (ECM) utilizes the upstream O2 sensor information to adjust the engine’s air-fuel ratio to maintain an optimum level (around 14.7:1.)

Oxygen sensors use heater elements to help them reach operating temperature. The ECM will log engine code P0155 when the heater element within the oxygen sensor heater circuit at Bank 2 Sensor 1 takes longer than usual to heat up. 

Note: “Bank 2” refers to the side of the engine that holds cylinder number 2, which could be either on the driver or passenger side, depending on how the engine is mounted. “Sensor 1” means it’s the upstream sensor.

Common symptoms

If left unresolved, a malfunctioning oxygen sensor heater circuit can lead to engine performance and drivability issues. 

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

• Flashing check engine light: Your vehicle’s check engine light gets triggered when the heating element inside your oxygen sensor heater circuit fails to bring the sensor to the right operating temperature. It may also occur if a bad coolant sensor relays erroneous coolant temperatures, and prevents your engine’s computer from taking cues from the O2 sensor to regulate the fuel mixture. 
• Weird noises from the engine: A malfunctioning oxygen sensor heater circuit may cause your engine to run lean (more oxygen, less fuel) or rich (more fuel, less oxygen.) In either case, your engine will produce strange sounds due to an imbalance in the air-fuel ratio. 
• Misfiring engine: A bad O2 sensor could cause an imbalance in the fuel supply, resulting in engine misfires. Your engine could also misfire due to faulty components in the oxygen sensor, including the heater wire or harness connector. 
• Spike in fuel consumption: If your O2 sensor heater circuit is faulty, it could misread the oxygen levels, causing the ECM to deliver too much fuel to the engine. So, if you’ve noticed a spike in the fuel consumption, it could be due to trouble code P0155. 
• Presence of black smoke or a rotten egg-like smell: If your vehicle runs on rich, excess fuel may not be completely burned in the combustion process. The excess fuel then escapes into the exhaust manifold and eventually out of your tailpipe, resulting in black smoke or a rotten egg-like smell emanating from the exhaust.

Can I still drive?

Ideally, you should avoid driving and have the issue rectified at the earliest possible. This is because the trouble code P0155 means an imbalance in the fuel to oxygen ratio, resulting in high fuel consumption, poor engine performance, and other drivability issues.  

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

P0155 causes

Depending on the vehicle’s make and model, several possible causes could trigger trouble code P0155. Here are the usual suspects: 

• A malfunctioning O2 heater circuit (Bank 2 Sensor 1)
• A faulty spark plug 
• A malfunctioning engine coolant temperature sensor
• High O2 sensor heater element resistance 
• A blown fuse connected to the O2 sensor heater circuit
• There are broken or bare wires leading to the oxygen sensor
• A damaged O2 sensor connector 
• The car battery voltage is low 
• The O2 heater element is faulty 
• The O2 sensor heater circuit is open, shorted to voltage, or shorted to the ground 
• The hose to the exhaust gas recirculation system module is disconnected

Diagnosis

Diagnosing the P0155 code will require technical know-how of the O2 sensor heater circuit functions. So, if you are unsure, you should have a reputable mechanic diagnose the issue. 

Here’s how your mechanic will diagnose the problem: 

• The mechanic will start the process by using an OBD-II scanner to collect the engine code stored by the engine control module.
• They will inspect the wiring connected to the O2 sensor heater circuit. Here, the mechanic will typically look for a bare or broken wire. 
• After visually inspecting the wiring, your mechanic will use the OBD-II scanner to review live data from the engine control module to check for any issues with the O2 sensor heater circuit. 
• They will then inspect data from the engine control module to check for any issues with the engine coolant temperature sensor.
• If the issue remains unresolved, your mechanic will check the engine control module to ensure it’s working properly. 

Common mistakes when diagnosing the P0155 code

When diagnosing trouble code P0155, your mechanic should make sure they inspect all the components, including the harness connector, the heater element, and so on. They should also look for broken or bare wires leading to the oxygen sensor. Failure to do so could mean replacing a perfectly good O2 oxygen sensor.

Possible repairs for P0155

Here are the possible repair solutions to fix trouble code P0155: 

• Fix or replace the bare or broken wire 
• Fix or replace the faulty coolant temperature sensor with a new sensor
• Fix or replace the O2 sensor
• Replace a faulty engine control module (although this is quite rare)