What does it mean?
OBD II fault code P0037 is defined as “ Oxygen Sensor Heater Control Circuit Low (Bank 1 Sensor 2), and is set when the PCM (Powertrain Control Module) detects a signal voltage from oxygen sensor #2 that does not fall within the predefined range for that sensor. Note that “Bank 1” refers to the bank of cylinders that contains cylinder #1, and that “Sensor 2” refers to the oxygen sensor that is located after (downstream of) the catalytic converter(s).
Regardless of the number of oxygen sensors present in a vehicle, each sensor is provided with a dedicated circuit to control the heating of the sensor element. Heating the element electrically brings the sensors into operation much sooner than would have been the case if the sensors were heated by the exhaust stream; by bringing the sensors into operation sooner, emissions are reduced significantly.
On most applications, the electrical resistance of an oxygen sensor will be 8 Ohms (or very close to it), and deviations of 10% of this value will set code P0037 and trigger the CHECK ENGINE light. Input voltages in all oxygen sensor heater control circuits should be equal to battery voltage; deviations of 10% will also set code P0037 and trigger the CHECK ENGINE light.
Note that the electrical resistance may differ on some applications- consult the manual on the exact values for the application being worked on. During normal (closed loop) engine operation, the signal voltage from the #2 oxygen sensor(s) will vary constantly, and in cases where the PCM detects no changes in the sensor signal voltage(s) for a period of about 8 continuous seconds, code P0037 can be set and the CHECK ENGINE light triggered.
Oxygen sensor #2 monitors the functioning of the catalytic converter, whereas sensor #1 monitors fuel/air metering. In practical terms, this means that sensor #1 measures the amount of oxygen in the exhaust stream (as a function of the combustion process) before it enters the catalytic converter, while sensor #2 measures how well the converter(s) copes with cleaning up the exhaust stream. NOTE: Some Japanese cars, such as Toyota, do not use conventional oxygen sensors.
Instead, these applications use AIR/FUEL RATIO sensors that are NOT interchangeable with conventional oxygen sensors. Consult the relevant repair manual for these applications for the correct testing/diagnostic/repair procedures. The image below shows the differences between conventional oxygen sensors and air/fuel sensors.
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 diagnostic procedure.