Oxygen Sensor And Air Fuel Ratio Sensor
O2 and A/F Sensor Diagnosis
Starting from 1996 all vehicles with OBDII system requires having at least two exhaust sensors. The sensors are located before and after the catalytic converter. The sensor before the catalytic converter is also called Air/Fuel (A/F) Ratio Sensor (Also called Upstream or Front Sensor) and it is used by the Engine Control Module (ECM) to adjust the air/fuel ratio. The rear O2 Sensor (Also called Downstream Sensor) located after the catalytic converter is used for the catalytic converter efficiency and monitoring.
Types of Oxygen Sensors
There are several types of O2 sensors, but the most common ones are the wide range oxygen.
OBDII vehicles require two exhaust sensors: one before and one after the catalytic converter. The A/F or Upstream (Front) O2 sensor before the catalytic converter is used by the ECM to adjust the air/fuel ratio and is in the S1 position (AFS B1 S1). The Downstream (Rear) O2 sensor after the catalytic converter is used for catalytic converter efficiency control and monitoring and is in the S2 position (O2S B1 S2). There are several types of oxygen sensors. The two most common are listed below: Narrow range oxygen sensor, typically called an oxygen (O2) sensor Wide range oxygen sensor, typically called an air/fuel ratio (A/F) sensor.
Oxygen Sensor Construction and Operation
The oxygen sensor sometimes referred to as heated oxygen sensor, HO2S or O2S, has been in service the longest. It is made of zirconia (zirconium dioxide), platinum electrodes, and a heater. There are two types in use: the Cup type and the Planar type. They vary slightly in construction and operation, but cannot be interchanged. The oxygen sensor produces a voltage signal based on the amount of oxygen in the exhaust of the engine compared to the atmospheric oxygen. A high exhaust oxygen content would indicate lean exhaust and result in low voltage output from the oxygen sensor. Low oxygen content indicates a rich exhaust and would result in a high voltage output from the sensor. The zirconia element has one side exposed to the exhaust stream while the other side is open to the atmosphere. Each side has a platinum electrode attached to the zirconium dioxide element. The platinum electrodes conduct the voltage generated. Contamination or corrosion of the platinum electrodes or zirconia elements will reduce the voltage signal output. Oxygen sensors signal circuits can be tested with a DVOM.
Oxygen Sensor Heater
The oxygen sensor cannot produce an accurate voltage signal until it reaches a minimum operating temperature of 750 degrees F (400 degrees C). It must reach that temperature quickly and stay at that temperature for effective operation. To help the oxygen sensor reach its operating temperature quickly, the ECM turns on current flow through a heating element inside the sensor. This element heats up as current passes through it. The ECM controls the circuit based on engine coolant temperature and engine load (determined from the MAF sensor signal). The oxygen sensor heater circuit uses approximately 2 amperes and is turned OFF once the engine reaches normal operating temperature. Typically, the heater will be turned ON at idle or in deceleration fuel cut conditions.
A/F Sensor Construction and Operation
The A/F sensor, sometimes referred to as the AFR sensor or air/fuel ratio sensor, looks like an oxygen sensor and serves the same purpose, but it is different in construction and operation. Instead of a varying voltage output, the A/F sensor changes its current (amperage) output in relation to the amount of oxygen in the exhaust stream. A detection circuit in the ECM uses this amperage to create a voltage signal that varies with the oxygen content of the exhaust gasses. At stoichiometry, there is no current flow, and the detection circuit outputs 3.3 volts. When exhaust oxygen content is high (lean), a positive current is produced, and the detection circuit outputs a voltage above 3.3V. When exhaust oxygen content is low (rich), a negative current is produced and the detection circuit outputs a voltage below 3.3V. These sensors detect A/F ratios over a wider range, allowing the ECM to more accurately control fuel injection and reduce emissions. Because of its nature, the A/F sensor signal circuit cannot be tested with a DVOM.
A/F Sensor Heater
A/F sensors operate at temperatures even hotter than O2 sensors, approximately 1200 degrees F (650 degrees C). The A/F sensor heater serves the same purpose as the O2 sensor heater, but there are some very important differences. A/F sensors require a much higher operating temperature than O2 sensors and must heat up to operating temperature very fast (within seconds) so: Some vehicles use an A/F Relay (turned on at the same time as the EFI Relay). A relay is required because the A/F sensor heater circuit carries up to 9.9 amperes (versus 2 amperes for oxygen sensor heater) to produce the additional heat needed by the A/F sensor. This heater circuit is pulse width modulated (PWM). When cold, the duty ratio is high. The heater may be ON under normal driving conditions to maintain proper A/F sensor operating temperature. Typically, when an A/F sensor heater DTCs are present, the ECM will turn OFF the A/F sensor heater as part of the fail-safe mode. The fail-safe mode will continue until the ignition switch is turned OFF. Because the proper operation of the A/F sensor depends on correct sensor temperature, the heater should always be checked when testing the sensor. A DVOM or oscilloscope can be used to test the A/F sensor heater operation.