P0031 – Oxygen Sensor Heater Control Circuit Low Bank 1 Sensor 1 (Toyota)

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By Reinier (Contact Me)
Last Updated 2023-11-21
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0031 Oxygen Sensor Heater Control Circuit Low Bank 1 Sensor 1
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Wiring, Electrical Connections

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Table of Contents

  1. What Does Code P0031 Mean?
  2. Where is the P0031 sensor located?
  3. What are the common causes of code P0031?
  4. What are the symptoms of code P0031?
  5. Get Help with P0031

What Does Code P0031 Mean?


SPECIAL NOTES: Although generic OBD II trouble codes such as P0031 – “HO2S Heater Control Circuit Low Bank 1 Sensor 1” can, and do affect all OBD II compliant vehicles, some vehicle makes and models are sometimes more susceptible to some generic trouble codes than most other vehicle makes and models under real-world driving and operating conditions. Therefore, this article will discuss generic trouble code P0031 as it applies to the Toyota models that are most commonly affected by it. END OF SPECIAL NOTES.

OBD II fault code P0031 is a generic trouble code that is defined as, “P0031 – HO2S [Heated Oxygen Sensor] Heater Control Circuit Low Bank 1 Sensor 1”, and is set when the PCM (Powertrain Control Module) detects an abnormal or unexpected voltage in the control circuits of the heater element of upstream oxygen sensor on bank 1. Note that “upstream” refers to the oxygen sensor that is located before the catalytic converter, and that “Bank 1” refers to the bank of cylinders on V-type engines that contains cylinder #1.

NOTE #1: Most modern Toyota vehicles use heated air/fuel ratio sensors (aka “Lambda sensors) instead of heated oxygen sensors. In a practical sense, air/fuel ratio sensors and oxygen sensors perform the same function, which is to measure the chemical composition of the exhaust stream, but they do this in vastly different ways. Moreover, while most oxygen sensor issues can be diagnosed with relatively simple equipment, most issues with air/fuel ratio sensors can only be diagnosed with oscilloscopes and other advanced equipment such as sensor simulators. As a practical matter, this means that oxygen sensors and air/fuel ratio sensors are not interchangeable.

NOTE #2: Be aware that despite the major technical differences between oxygen sensors and air/fuel ratio sensors, many online “How to…” guides and self-help articles aimed at the general motoring public use the terms “oxygen sensor” and “air/fuel ratio sensor” interchangeably, which is confusing and misleading from a diagnostic perspective. Therefore, this article will use the term “air/fuel ratio sensor” throughout to avoid confusion and misunderstandings that could lead to potentially expensive misdiagnoses, but note that the finer details of how air/fuel ratio sensors work fall outside the scope of this article,

In order to understand the role that upstream air/fuel ratio sensors play in how a PCM controls the air/fuel mixture on modern vehicles, it is important to understand the distinction between upstream and downstream sensors.

As their name suggests, “upstream” sensors are located before, i.e., upstream of a catalytic converter, while “downstream” sensors are located after, i.e., downstream of a catalytic converter.  However, the most important distinction between upstream and downstream sensors is the fact that even though the sensors are identical, upstream sensors react in almost real-time to changes in the composition of the exhaust stream, while downstream sensors have the sole function of monitoring the efficiency of a catalytic converter. Therefore, the signal voltage of downstream sensors remains fairly constant, and only changes markedly when a) the efficiency of a catalytic converter changes over time, or b) when large hydrocarbon loads temporarily overwhelm a catalytic converter’s ability to convert harmful emissions into less toxic substances

In terms of operating principles, and assuming that both the larger engine management system and emission control systems are fully functional, the upstream air/fuel ratio sensor samples the exhaust stream continuously when the engine is running. For example, if the engine is running at a steady speed under cruising conditions, the oxygen content of the exhaust stream will typically remain fairly constant since under cruising conditions the throttle opening does not change much, if at all. This means that under cruising conditions, the upstream oxygen sensor will report no significant changes in the oxygen content of the exhaust stream, and the PCM will, as a result, make no adaptations to the volume of fuel that the fuel injectors inject into the cylinders.

Conversely, if a driver makes repeated throttle inputs during city driving conditions, the upstream air/fuel ratio sensor will react to each throttle input by reporting either a lean or rich running condition. In response, the PCM will either add or subtract fuel from the air/fuel mixture to return the air/fuel mixture to a stoichiometric ratio, i.e., a mixture of one part of fuel to every 14.7 parts of air.

The above should suffice to illustrate the basic function of upstream air/fuel ratio sensors, but code P0031 is not so much about the overall operation of the upstream air/fuel ratio during closed-loop* operation, as it is about its operation before it enters closed-loop operation. More precisely, though, code P0031 refers to a condition that greatly increases the time it takes for an upstream air/fuel sensor to enter closed-loop operation.

* Closed-loop operation begins when a control module uses only input data from one or more sensors to manage and/or control a particular system, which in this case, is the fuel delivery system on a modern engine.

Here is what the above means in practice-

Until comparatively recently, all air/fuel sensors and oxygen sensors depended on the heat of the exhaust stream to heat them to a point where they could begin to react to changes in the exhaust stream. As a practical matter, this could take as long as ten minutes or more in freezing temperatures, meaning that the PCM could exert no control over the composition of the air/fuel mixture during this time since the air/fuel ratio sensors were not yet working.

Since this situation defeated much of the purpose of the then-current exhaust emission regulations, regulatory authorities began requiring car manufacturers to develop technologies that reduce sensor warm-up times to reduce harmful exhaust emissions during cold engine starts. Car manufacturers responded to what amounted to threats of legal action by several regulatory authorities by developing air/fuel ratio sensors that contained dedicated heating elements that came into operation when the ignition was switched on.

In practice, the first iterations of heated air/fuel ratio sensors reduced sensor warm-up times by about 50%, but subsequent refinement reduced this further by large margins. In fact, most, if not all modern air/fuel ratio sensors now reach their operating temperatures in well under 20 seconds in sub-zero ambient temperatures and only about 4 or five seconds in warm to hot weather.

The practical effect of reduced sensor warm-up times is that in many cases, air/fuel ratio sensors are at their optimal operating temperatures even before combustion in cold engines has stabilized. This means that, essentially, cold engines no longer emit vastly more harmful substances than hot engines since very short sensor warm-up times now allow a PCM to exert a greatly increased measure of control over fuel enrichment strategies during cold engine starts.

It should be noted that the PCM deactivates the heating elements in air/fuel sensors either when the sensor reaches its operating temperature or when the fuel delivery system enters closed-loop operation, which is subject to the engine coolant temperature.

From the above, it should be clear that upstream air/fuel ration sensors play a critical role in effective emission control. Therefore,  when the PCM detects an abnormal or unexpected signal or voltage in the control circuit(s) of an air/fuel sensor, it will recognize that it cannot control the fuel delivery system (and by extension, the emission control system) effectively and it will set P0031 and illuminate a warning light that may or may not flash.

Where is the P0031 sensor located?

This image is a simplified schematic diagram of the first half of a typical exhaust system. Note the location and position of the sensor indicated by the red arrow; this is the legally mandated position of an upstream air/fuel sensor shown relative to the catalytic converter, which is circled in green. The yellow arrow indicates the direction of flow of the exhaust stream.

Note that in all cases on all vehicle makes and models, the upstream air/fuel or oxygen sensor will always be located upstream of the catalytic converter, but be aware that the actual locations of upstream sensors in exhaust systems vary between vehicle makes and models.

Note also, that since many exhaust systems accommodate several sensors apart from air/fuel ratio or oxygen sensors, it is strongly recommended that reliable service information for the affected vehicle be consulted to locate and identify the affected sensor correctly.

What are the common causes of code P0031?

The term “Circuit Low” means that the PCM is receiving an input or feedback signal voltage from the upstream air/fuel ration sensor’s heating element’s control system that is lower than expected based on inputs from other sensors such as the engine coolant temperature sensor. As a practical matter, this means that the heating element is not working as expected.

While this condition can have many possible causes in a wide range of Toyota vehicles, the most common cause of this code on Toyota vehicles involves the location of the upstream air/fuel ratio sensor on the exhaust manifold on many engines. This location is subjected to extreme heat that results in the degradation of both electrical connections and electronic components inside the sensor itself, as well as in the electrical connector that connects the sensor to the vehicle’s electrical system.

The first condition is self-explanatory, but in the second instance, the sensors’ wiring harness is often not routed in a way that protects it from the extreme heat close to the exhaust manifold. As a result, the wiring’s insulation degrades over time, which often causes short circuits or poor continuity that causes abnormal electrical resistance values in the sensor’s wiring. Note that wiring insulation does not have to be visibly damaged or cracked to cause abnormal resistances; in many cases, cracks in wiring are microscopic but big enough to cause issues when currents pass through the conductor.

Having said the above, other possible causes of code P0031 could include one or more of the following-

  • Malfunctioning air/fuel ratio sensor’s heating element for reasons that do not involve degraded wiring
  • The use of a substandard or unsuitable aftermarket air/fuel ratio sensor
  • Damaged wiring as the result of routine servicing and maintenance work
  • Partially melted or damaged fuses that could cause abnormal resistance values in the wiring of the sensor’s heating element
  • Damaged or malfunctioning heating element in the affected air/fuel ratio sensor
  • Damaged or corrupted sensor heating driver circuits in the PCM; this could include a failure of the PCM to supply a proper ground connection to the heating element in the affected air/fuel ratio sensor

What are the symptoms of code P0031?

Common symptoms of code P0031 are largely similar across all applications and could include one or more of the following-

  • Stored trouble code and an illuminated warning light; in some cases, this might be the only discernible symptom of this code when no other codes are present
  • Poor idling quality when the engine is cold; this will usually resolve itself when the hot exhaust gas heats the affected air/fuel ratio to its optimal operating temperature
  • The engine may stumble or hesitate upon acceleration when the engine is cold
  • The engine may run roughly or exhibit misfire-like symptoms when the engine is cold
  • Varying degrees of power loss may be present when the engine is cold
  • In rare cases, and depending on the nature of the problem, the affected vehicle may be locked into a fail-safe or limp mode
  • One or more readiness monitors may not initiate or run to completion
  • The affected vehicle will not pass an emissions test for as long a code P0031 remains unresolved

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