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P0031 Code: Meaning, Symptoms, Causes, Diagnostics, And Fixes

Error code P0031 refers to a diagnostic trouble code for HO2S Heater Control Circuit Low. (Bank 1 Sensor 1). This sensor is located in front of the catalytic convertor. There is a second sensor, sensor 2, located behind.

The code means that the heated oxygen sensor 1 has low voltage passing through it. Once you switch on your engine, it takes time before the sensors are at optimum operating temperature. They rely on the hot exhaust gases to bring them to this optimum temperature. However, the ECM will need data immediately to control the intake of air and fuel to the engine. The heated oxygen sensors thus have a heater inside them.  When this heater has a problem, the code appears.

When the P0031 code is triggered, you should check the oxygen sensor.
When the P0031 code is triggered, you should check the oxygen sensor.

P0031 Code Definition

  • P0031 Generic: Oxygen Sensor Heater Control Circuit Low Bank 1 Sensor 1
  • P0031 Subaru: Faulty Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1
  • P0031 Nissan: Heated oxygen sensor (HO2S) 1 Bank 1 – heater voltage low

What Does P0031 Mean?

P0031 OBD2 code means that the ECM has detected a low voltage across the heated oxygen sensor. This low voltage is caused by the heater element or the sensor not functioning as it should. The heater enables the sensor to start working immediately after the engine is turned on. The sensor sends data to the ECM

The ECM relies on the sensor to control the amount of air and fuel delivered to the engine. The sensor depends on the hot exhaust gas to fire it up and get accurate readings. The heater circuit does this work when the engine is warming up. Once the ECM receives readings from sensor 1, it adjusts the intake of air and fuel to control emissions and make the engine run efficiently.

P0031 code may occur in other models including Toyota, Subaru, and Nissan.

What Are The Symptoms Of The P0031 Code?

  • Check engine light turning on (Service engine soon in some models)
  • Drivability issues
  • Engine hesitation
  • Reduced engine performance
  • Failsafe mode

What Are The Causes Of P0031?

There are several possible causes of P00031 code:

  • Faulty connectors or heater circuit
  • Faulty heated oxygen sensor
  • Blown fuses
  • Failed battery input
  • Loose wiring
  • Faulty ECM (this is rare and should be explored as a last resort.)

How Serious Is P0031 Code?

The P0031 code is a moderate issue in terms of severity. You still be able to drive around with this code, but it is important to have it checked and fixed. You’ll experience some challenges driving with this code, such as drivability and rough running. This might damage other parts of the vehicle. Driving with this code may lead to lower fuel economy and more emissions. In some jurisdictions, it is a traffic offence to drive with the check engine light on. You may not pass an emission test if the code P0031 still present.

How To Diagnose And Fix The Code P0031

Tools you’ll need

 We use an OBD2 scanner to diagnose the P0031 code.
We use an OBD2 scanner to diagnose the P0031 code.

How To Diagnose The Code P0031

To diagnose the P0031 code, you’ll need an OBD 2 scan tool. Once you diagnose the P0031 code, you should attempt to reset it. After the reset, a short test drive is advisable to check if the code returns. Sometimes, the code will go away. If it doesn’t go away after the test drive, then you have to fix it.

How To Fix The Code

  • Using the OBD 2 scan tool, check the fault code. You should do this with the key on and the engine turned off. Reset the code and perform a test drive. If the fault code is still present, carry out the following steps.
  • Perform a thorough visual inspection around the Bank 1 sensor 1 area. Check for any loose wiring. If you notice anything out of place, repair it and recheck the code. The exhaust is very hot which can damage wiring. If wiring is intact, proceed to the next step.
  • Using a digital multimeter, check that the battery provides 12 volts to the heater element. If less than 12 volts or no voltage at all is present, check the circuit and repair it. Check if any blown fuses are the problem or for a broken circuit.
  • The next step is to check the grounding resistance. The resistance should not be infinite. If it is, there is an open circuit that needs to be repaired.
  • If you have followed these steps, and the fault code is still present, the heated oxygen sensor 1 is the problem. Replace the sensor with a new one, then rescan to check if the code has gone.
  • In most cases, the code will have cleared by this step.  If it hasn’t, the problem is a faulty ECM. Your mechanic will diagnose the ECM and advise if it needs to be repaired or replaced.

Notes: The wiring and grounding to the sensor should be done properly after you have changed the sensor or the heater circuit. Failure to do this will lead to future problems.

Common Mistakes While Diagnosing Code P0031

One of the common mistakes people do is to replace the heated oxygen sensor 1 on bank 1 immediately the code appears. This can lead to high costs whereas the sensor might not be the problem. The heater circuit is mostly where the problem is. Many motorists have changed the sensor only for the code to persist.

Another mistake is replacing the sensor with an old sensor. This is not advisable. If you have confirmed that the sensor is the culprit, always replace it with a new one. 

Vehicles with a high mileage might have a sensor problem that lasts only for a short period of time. This can occur during start-up or any other time. It will normally resolve itself.

Tips to Avoid P0031 in The Future

The exhaust is very hot. Burnt wiring is the most likely cause of the P0031 code. Rerouting wrings away from the hot exhaust will avoid this code in the future.

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