P0155 – O2 Sensor Heater Circuit

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The P0155 OBD2 code indicates the heating element inside the upstream bank 2 oxygen sensor takes too long to heat up. When this happens, your vehicle spends too long-running on a rich fuel mixture. The results are a rough ride and elevated emissions, causing you to fail your next emissions test.

Fixing the P0155 trouble code is often a quick and inexpensive repair. At worst, you’ll need a new oxygen sensor. In many cases, though, the problem is a more simple loose wire or a blown fuse. Starting your diagnosis by checking the small fixes can save you money and hassle in the long run.

The heating element inside the upstream bank 2 oxygen sensors is shown by P0155 OBD2 code
The P0155 OBD2 code indicates the heating element inside the upstream bank 2 oxygen sensor is taking too long to heat up

P0155 Code Definition: O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)

What Does P0155 Mean?

The P0155 OBD2 code triggers when the heater element in the oxygen (O2) sensor takes longer than it should warm up. The sensor itself may be reading fine otherwise, and you’ll often see this reflected in live data readings from your vehicle.

The oxygen sensors have to be at operating temperature to maintain the air-to-fuel mixture properly. They use heating elements to speed this process, which minimizes the time you spend driving with a rich fuel mixture. 

Every vehicle has an ideal air-to-fuel ratio that will allow the engine to run at peak efficiency. When the mixture is too rich, that means it has more fuel in it than it should. This problem makes the engine run too hot and can build up in the catalyst and lines, leading to clogs. 

Driving with a rich fuel mixture will cause you to burn more fuel than your engine needs, which increases the fuel vapors in the exhaust. If it goes on too long, it can damage other components of the fuel system. This is why it’s so essential that the heating element function properly.

The powertrain control module (PCM) or engine control module (ECM) tracks how long it takes the oxygen sensor to reach operating temperature. P0155 is set when the ECM or PCM determines too much time has elapsed before the oxygen sensor starts switching.

There are multiple oxygen sensors in your engine. P0155 triggers from an issue with the upstream oxygen sensor on bank 2. This code will also trigger if the PCM detects a high resistance or short circuit in the bank 2 upstream oxygen sensor. 

As we said above, the oxygen sensor can still function even with a bad heating element. Unfortunately, the heating element of an oxygen sensor can’t be removed. If the heating element has an internal short or has otherwise failed, you’ll have to replace the entire sensor. 

What Are The Symptoms Of The P0155 Code?

  • Illumination of the check engine light
  • Rough running engine, especially on start-up
  • Reduced fuel economy
  • Sulfur smell or the black smoke from the exhaust

What Are The Causes Of P0155?

  • Faulty bank 2 upstream O2 sensor
  • Faulty or loose wires around O2 sensor
  • Blown fuse in the O2 sensor circuit
  • Shorts or open grounds in the O2 sensor wiring
  • High resistance in the O2 sensor heater element
  • Internal short in O2 sensor heater element
  • Faulty engine coolant temperature sensor
  • Faulty PCM or ECM (less likely)

How Serious Is The P0155 Code?

The P0155 trouble code is of moderate severity. It’s safe to drive your car for a little while with this code active. However, you’ll have to contend with elevated emissions and reduced gas mileage until you fix the problem. Because of that, it’s a good idea to make repairs as soon as possible. 

How To Diagnose And Fix The P0155 Code

Tools you’ll need:

  1. Read and clear all present codes using your OBD2 scan tool. Do a road test, then re-scan to see if the code comes back. 
  2. If there is a problem with the engine coolant temperature sensor, you’ll see additional OBD2 trouble codes, such as P0118. Address those codes first, then clear the codes and see if the P0155 code returns. 
  3. Use a multimeter to verify that the oxygen sensor is getting battery voltage. Disconnect the connector to the harness, then turn your vehicle’s ignition on without starting the engine. If the sensor isn’t receiving a voltage, the problem is likely a blown fuse. Locate and replace the fuse for the upstream O2 sensor circuit.
  4. Visually inspect the wires around the oxygen sensor and wiring harness. Check for damage on the terminals as well as fraying and other damage to the wires themselves. Replace wires as necessary, then ensure all connections are secure. 
  5. Locate the engine ground and check the connections. Also, inspect the ground for corrosion. Clean away any corrosion you see, tighten any loose connections, and then clear all trouble codes, test drive, and re-scan. 
  6. Test the upstream bank 2 oxygen sensor using a multimeter with the engine running. The voltage from the sensor should fluctuate between around 100 mV and 900 mV. If you read a steady voltage, replace the oxygen sensor. 
  7. In rare cases, a P0155 trouble code indicates a problem with the ECM or PCM. If the code still will not clear after you’ve followed all the diagnosis steps above, take your vehicle to a mechanic. They can test and diagnose any issues with your engine computer and advise you of the best next steps. 

Common Mistakes To Avoid While Diagnosing The P0155 Code

Don’t immediately replace the O2 sensor when you see the P0155 trouble code. The problem could very well be with the wiring around the sensor. Conduct a full, thorough diagnosis before you replace any components.

Tips To Avoid P0155 In The Future

In many cases, the P0155 trouble code is the result of bad wiring. Always make sure to tighten connections firmly after you make repairs, and check the wiring as part of your regular maintenance. Vibrations can dislodge wires that seemed secure, and high heat inside the engine block can melt or damage wires. A quick visual inspection can let you fix these small issues before they become big problems.

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