Oxygen (O2) Sensor

Oxygen sensors are used to measure the amount of oxygen in the exhaust gases. Depending on emissions package either 2 or 4 sensors are used. If vehicle is a non-California emissions package, 2 sensors are used. If vehicle is a California emissions package, 4 sensors are used. All 4-cylinder engines use 2 sensors regardless of emissions package. O2 sensors are placed upstream and downstream of catalytic converter. Sensors upstream of catalytic converter are referred to as 1/1. Sensors downstream of catalytic converter are referred to as 1/2. If vehicle has more than one sensor upstream of catalytic converter, they will be referred to as 1/1 (left bank) and 1/2 (right bank). If vehicle has more than one sensor downstream of catalytic converter, they will be referred to as 2/1 (left bank) and 2/2 (right bank).

O2 sensors produce their own voltage which varies from zero to one volt. The voltage varies in correlation to oxygen content in exhaust gas. If a lot of oxygen is present in exhaust gas, voltage produced will be near zero volts. If very little oxygen is present in exhaust gas, voltage produced will be near one volt. The PCM uses this voltage to adjust air/fuel mixture. O2 sensors have an opening (near sensor wiring harness) to monitor air outside the sensor. The sensors compare outside oxygen content with oxygen content in the exhaust gas and produce a voltage accordingly.

For O2 sensors to produce a voltage they must maintain an operating temperature of 930-1100°F (499-593°C). O2 sensors use a heater circuit to heat up quickly and maintain operating temperature. The heater circuit is a 12-volt circuit. Power for the heater circuit is controlled through the Auto Shutdown (ASD) relay on certain non-California emissions packages. The ASD relay is controlled by the PCM. On other non-California emissions packages, heater circuit is controlled through O2 sensor heater relays. These relays are also controlled by the PCM. On California emissions packages, heater circuit is controlled by 2 O2 sensor heater relays. On all emissions packages, the O2 sensor heater is a Positive Thermal Coefficient (PTC) element. As temperature increases, resistance increases. When temperature decreases, resistance decreases.

O2 sensors work by the same principal, however a sensor used upstream of a catalytic converter cannot be interchanged with a sensor used downstream of a catalytic converter. There are physical differences preventing interchangeability. An upstream O2 sensor is used by the PCM to monitor oxygen content before the catalyst. The O2 sensor voltage input is used by the PCM to adjust air/fuel mixture to make the downstream O2 sensor voltage correct. The upstream O2 sensor is also used to determine catalyst efficiency. A downstream O2 sensor is also used by the PCM to monitor oxygen content after the catalyst. The O2 sensor voltage input is used by the PCM to determine air/fuel mixture adjustments as the downstream oxygen content changes. As the air/fuel mixture is changed, the PCM monitors upstream O2 sensor voltage and changes fuel delivery until upstream O2 sensor voltage changes enough to correct downstream O2 sensor voltage. The downstream O2 sensor is also used to determine catalyst efficiency.