Error Code P0051 is defined as Heated Oxygen Sensor (HO2S) heater control circuit low bank 2, sensor 1. This error code refers to a problem with the heater element circuit of the HO2S, specifically low voltage.


Error Code P0051 is a generic term referring to a problem in the Oxygen Sensor Heater Control Circuit, located in Bank 2, Sensor 1, which is in front of the catalytic converter. It is triggered when the PCM (powertrain control module, also known as ECM or engine control module) determines a low system voltage in the heater control circuit referencing at Bank 2, Sensor 1. The job of the heater circuit of the HO2S is to warm the sensor before the engine reaches its operating temperature for a precise reading.

This error code may also be referenced as air/fuel ratio sensor, sine it’s the case in some vehicles. It reads the amount of oxygen in the exhaust gas and compares it to the outside air. The PCM automatically adjusts its air/fuel ratio that goes into the engine. The sensor becomes less effective when the temperature of the exhaust gas is too low, which is why it comes with a heater that gets activated to help get better readings from the A/F oxygen sensor. Essentially, this code means heater circuit’s resistance is lower than normal. Most of the time, the resistance level must be below 0.8 A to trigger this error code.

This error code is similar to P0031, P0032 and P0052.

Common Symptoms

Aside from the Check Engine light being activated, this error code causes poor fuel economy and rough riding.

Possible Causes

As with many error codes that concerns the HO2S, P0051 is caused by water getting into the HO2S connector, causing the sensor’s fuse to blow up. Other common causes include:

  • Short in the sensor of the heater circuit
  • Failed oxygen sensor heater
  • Broken connection, damaged wiring, or frayed wiring to the sensor and/or relay
  • Broken PCM (or ECM)

How to Check

It is important to conduct a proper diagnosis before doing any repairs. With that, you will need to conduct a proper inspection of the connectors and the wirings that leads to the sensor. If the engine system has a heater relay and fuse, you will have to check those as well. For a more thorough diagnosis, you will need to use a digital volt ohm meter, especially to:

  • Check for 12V of heater circuit feed (unplug the sensor and then check the wiring connector with its measurement).
  • Test ground for continuity
  • Measure the heater circuit’s resistance (using the sensor)
  • Measure the wiring’s resistance and voltage

You may have to refer to your manufacturer’s specifications when it comes to the specific volt and ohm measurements of your vehicle. For some Toyota models, P0051 is set when heater circuit’s resistance hit lower than 0.8A.

As with other error codes, mechanics diagnose this error code with the use of a professional-grade scan tool; they take note of freeze frame data associated with the code and check it through a test drive.

After confirming the code, they will conduct a thorough visual inspection of the connectors and wirings of the sensor.

If the wiring is in good condition, the mechanic will then use their scan tool to monitor the live data from Bank 2, Sensor 1.

How to Fix

With that being said, the most common fix for this error code is to replace the air/fuel oxygen sensor 1 on Bank 2, found on the side of the engine that doesn’t have cylinder #1.

Other obvious fix include:

  • Fixing the issues in the connection
  • Reattachment of connectors
  • Repair open or shorted wiring
  • Repair the fuse of the heater circuit
  • PCM (or ECM) replacement

It is important to never skip a step in the diagnosis to avoid unnecessary repairs or replacement.

To check for the integrity of the operation of the concerned components, it is important to monitor the sensor data through the scan tool’s live data stream. The sensor shouldn’t be replaced after it is tested.

When looking to replace, make sure you get recommended components from your dealer. Aftermarket components (such as sensors) can be of poor quality or less reliable (though not always, but more often). You can also check whether your replacement part for this error code may fall under federal emission warranty, so check it with your dealer.