What does it mean?
OBD II fault code P0051 is defined as “HO 2 S (Heated Oxygen Sensor) Heater Control Circuit Low (Bank 2 Sensor 1)”, with “Bank 2” referring to the bank of cylinders opposite to the cylinder #1 on engines with two cylinder heads, and “Sensor 1” referring to the oxygen sensor located before the catalytic converter(s), i.e., upstream of the converter(s).
OBD II code P0051 is most commonly set when the PCM (Powertrain Control Module) detects a signal voltage or a resistance level from the oxygen sensor control circuit that does not fall within predefined parameters. On most applications, the sensor heater circuit voltage should closely match the battery voltage, and a deviation from battery voltage of around 10% will lead to the PCM generating and storing a fault code, and illuminating the CHECK ENGINE light.
Note that two other conditions can also cause code P0051 to be stored- On most applications, the electrical resistance in the circuit would typically be around 8 Ohms, and a deviation of around 10% from this value will result in a code being stored. A code can be stored when the PCM does not detect a change in the oxygen sensor reading for periods of more than 8 continuous seconds.
NOTE: The time required for static (constant) readings to cause a code to be stored may differ between applications, so refer to the repair manual for exact values. In order for any engine to work at peak efficiency, it requires an air to fuel ratio that allows all of the fuel in the mixture to be combusted without any excess air remaining after the combustion process is complete, or with additional air required to complete the combustion process.
This ratio is known as the “stoichiometric” mixture/ratio, and it refers to a fuel to air ratio required by an engine under an unvarying load. There is no ideal fuel to air ratio that applies to all engines under all conditions, and many factors, such as driving style and driving conditions can temporarily swing the ideal mixture for a particular engine towards either a lean, or rich condition.
However, the PCM on any particular application collects data from various sensors (including the oxygen sensor(s)), to calculate and deliver the ideal air/fuel mixture to suit the reigning operating conditions.
The role of the upstream oxygen sensor(s) in this is to measure the amount of oxygen in the exhaust stream, which value is used by the PCM to make adjustments to ignition timing, VVT/VCS systems (where fitted), and injector pulse widths to maintain an optimum fuel delivery strategy, thus keeping the engine performing at peak efficiency.
NOTE #1: Most modern oxygen sensors are heated electrically, which makes them responsive much sooner than if they were heated only by the exhaust stream, thus reducing emissions, as well as the time required for the PCM to enter a closed-loop mode. NOTE #2: Some applications, such as some Toyota and other Japanese-made vehicles, use AIR/FUEL SENSORS instead of conventional heated oxygen sensors.
Refer to the relevant manual for code setting parameters and testing procedures for these applications, since the two types of sensors are NOT interchangeable. The image below shows some of the differences between a typical heated oxygen sensor, and a typical air/fuel sensor.
Note: Circuit Low Input codes are often the result of low battery voltages (that can have many possible causes), bad connections across electrical connectors or previously repaired wiring, as well as corrosion in electrical connectors.
Other possible causes of low input voltages include poor installation of aftermarket components, poor quality aftermarket components like fuses, relays, and switches, and modification of the electrical system that could include the use of conductors that are not rated for use in a particular application. However, poor connections often result in high resistances in some parts of the circuit, which is why it is important to perform resistance and continuity checks during the diagnos