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If the Check Engine Light is illuminating your dashboard and the OBD-II reader is displaying P0155, you probably want to know what this code means. If you’re only interested in the code’s most basic definition, we’ll cut to the chase: it’s “O2 Sensor Heater Circuit Malfunction (B2S1)”. If, on the other hand, you want to know more, including the code’s meaning, potential causes and exact diagnostic and repair steps, keep on reading.

car oxygen sensor

The Meaning and Cause of the P0155 Code

The P0155 Diagnostic Trouble Code (DTC) is a generic powertrain code, meaning it applies to all vehicles equipped with the OBD-II system. Although generic, the specific repair steps for this DTC will vary from model to model (of the vehicle).

Back to that definition: “O2 Sensor Heater Circuit Malfunction (B2S1)”. To understand what the P0155 means, we need to break this down. Obviously, the 02 sensor part refers to the oxygen sensor; circuit malfunction is clear enough – something’s not working properly in the oxygen sensor circuit. As for those letters and numbers, B2 and S1, they refer to the bank and the sensor, respectively. So the B2 refers to the Bank 2, but to find it, you have to know where the Bank 1 is. Luckily, that’s not too hard, as the Bank 1 refers to the side of the engine that has the #1 cylinder; Bank 2 is therefore on the opposite side of the engine. If your vehicle has a four-cylinder, there is only one bank, which means you wouldn’t get this code. As for S1 part of the definition, it refers to sensor #1.

But what causes the P0155 code?

There can be a number of reasons for this DTC to show up, but the most common causes include:

  • Internal short or bare wire leading to the O2 sensor
  • Faulty oxygen sensor on Bank 2
  • O2 sensor heater element has a high resistance
  • O2 sensor heater element has an open circuit
  • Damaged wiring/loose connections
  • Short or open to the ground in the wiring harness
  • ECT sensor not working properly
  • Power control module not working properly (rare).

So basically, when your OBD-II reader/scanner reveals the P0155 DTC, it means that when your vehicle’s powertrain control module had tested the O2 sensor’s heater circuit on Bank 2, it noticed an excessive resistance or short in the heater element. This translates to O2 sensor taking longer to heat up and enter closed loop.

The Symptoms of the P0155 Code

Normally, a failed O2 sensor leads to poor fuel economy, however, it’s entirely possible to experience no symptoms at all when the P0155 code shows up, except for the illuminated Check Engine Light. Still, most drivers will experience a combination of the following symptoms:

  • Check Engine Light is on
  • Decreased fuel economy
  • The engine may run rough
  • There may be a sulfur smell coming from the exhaust
  • There may be black smoke coming out of the exhaust.

Diagnosing and Repairing the P0155 code   

In most cases, when the P0155 DTC shows up, the O2 sensor needs to be replaced. However, this is not always required, as sometimes the problem can lie in the catalytic converter, or an exhaust leak or damaged wiring or connections. For this reason, it’s crucial to inspect your vehicle thoroughly and pay attention not only to the O2 sensor, but to the other components of the car as well. Although the P0155 code is not one of the serious malfunctions, it’s still important to take your time to properly inspect, repair and test the vehicle. So, unless you have quite a bit of automotive knowledge, it’s a good idea to let a professional fix your car.

In any case, here is what you or your mechanic should do to inspect your vehicle and fix the P0155 DTC:

  • Plug the OBD-II scan tool in the DLC port and note all stored codes and freeze frame data. Having this information is crucial as it can reveal the condition of the vehicle at the time when the P0155 code first showed up.
  • Inspect the wiring that leads to the O2 sensor. Look for damaged or bare wires and repair or replace things as necessary.
  • Test the O2 sensor for proper voltage using a multimeter (of course, consult the instructions from your car repair manual first).
  • If you find a damaged sensor or wiring, repair it and use the scan tool to view live data in order to ensure the O2 sensor is working as it should. If the O2 sensor appears to be in good condition and is running as it should, leave things as they are and proceed to the next step.
  • Check the engine ground and look for corrosion, damage and loose connections. Remove corrosion, repair and tighten things as necessary and restart the diagnostic process.
  • If the ECT sensor is not working as it should, the power control module will store this code too, in which case you’ll want to deal with it.
  • Sometimes, the power control module itself can be the problem. In this rare case, advanced inspection and diagnosis is necessary, which is not a job for a layman.

To properly diagnose and repair the P0155 code, a thorough inspection is a must. Often, people will replace the O2 sensor before inspecting other components which just leads to higher repair costs and frustration.

To conclude, the following repairs can fix the P0155 DTC:

  • Fixing broken/replacing bare wires
  • Replacing the O2 sensor
  • Replacing the ECT sensor
  • Rarely, replacing the power control module.

car part oxygen sensor

Additional Information about the P0155 Code

Because the code P0155 doesn’t pose danger to the driver, it’s not considered a serious DTC. It is, however, a moderately serious code. This is because it often causes drivability issues and poor fuel economy. Plus, the longer you wait to fix it, the higher the repair costs will be. Needless to say, it’s wise to deal with this code as soon as you notice it.

If you don’t own an oxygen sensor set and a propane tank, removing the oxygen sensor can be tricky. For this reason, it’s a good idea to let a professional diagnose and repair the P0155 code.

Code Explanations

These links will provide more detailed insights into what code means in your vehicle:

P0303 Code
P0456 Code
P0430 Code

Be sure to also read our essential guide to the best OBD2 Scanner.

Sources:

  1. P0155 OBD-II Trouble Code – YourMechanic
  2. How to Read and Understand OBD Codes – wikiHow

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