The heated Oxygen (O 2 S) sensor compares the oxygen content of the surrounding outside air with the quantity of oxygen in the exhaust gasses and produces a corresponding voltage signal as an input to the Engine Control Module (ECM). If the mixture is too rich, a voltage signal indicating low oxygen content is produced ranging from approximately 0.6 to 0.9 volts. If the mixture is too lean, a voltage signal for high oxygen content is produced ranging from about 0.0 to 0.3 volts. During the transition period from rich to lean or lean to rich, the voltage fluctuates between 0.6 to 0.9 volts and 0.0 to 0.3 volts.
Because of the constant and sudden voltage fluctuation, the O 2 S sensor output can correspond to the ideal or stoichiometric mixture, however the mixture control is not held constant, rather the control constantly fluctuates back and forth in a window between the mixture too rich and mixture too lean conditions. When the engine is at operating temperature, the control frequency during idle is a minimum of 30 cycles per minute (0.5 Hz.). As the engine speed is increased to 2500-2800 RPM, the control frequency must be at least 60 cycles a minute (l Hz.). A cycle is a voltage swing from the highest value to the lowest value and back to the highest value again.
If the voltage does not fluctuate, or the sensor responds too slowly (lazy), the following conditions are possible causes:
The time interval between cold start (open loop) and O 2 S sensor control (closed-loop), which requires an oxygen sensor temperature of about 5720°F (3000°C), is affected by the following variables:
With a properly functioning O 2 S sensor, the actual control learning values can be checked. Various malfunctions such as an intake air leak or faulty fuel injectors can cause a change in the fuel mixture. These malfunctions are recognized by the O 2 S sensor and corrected by the O 2 S sensor control by adjusting the fuel injection time controlled by the ECM. Fuel injection time that deviates from the programmed basic fuel injection time indicates a malfunction and will likely store a Diagnostic Trouble Code (DTC) and may activate the Check Engine/Malfunction Indicator Light (MIL).Fuel Mixture Control
- Turn the ignition switch to the OFF position.
- Disconnect Heated Oxygen sensors electrical connector.
Connect a suitable Digital Multimeter (DMM) to terminal No. 4 and ground of the connector from the O 2 S sensor. Measure the resistance to check for continuity between terminal No. 4 of the O 2 S sensor signal wire at the O 2 S sensor connector and engine ground. If continuity is not present, replace the heated O 2 S sensor.
- If continuity is present, check for a reference voltage between terminal No. 4 and engine ground. Using the 2 volt range of the DMM, switch the ignition ON and measure the voltage. The voltage should be 350-450 mV. If the voltage reading is not within specification, check the wiring from the ECM and the ECM.
To check the heated circuit of the O 2 S sensor:
- With the ignition OFF, check the resistance between terminal Nos. 1 and 2 of the O 2 S sensor connector. At room temperature the resistance should be 0.0-24.2 ohms.
- With the ignition OFF and the O 2 S sensor disconnected, use a suitable DMM to check the voltage between terminal Nos. 1 (+) and 2 (-). Start the engine and allow the engine to idle. The reading should be about 12-14 volts. If the voltage is not present, check the fuses and electrical wiring.
- Disconnect the heated oxygen sensor harness electrical connector.
- If installed, open the tie wrap.
- Using a suitable tool, remove heated oxygen sensor.
- Installation is in reverse order of removal noting the following:
- Install all tie wraps in their original locations, to prevent the wiring from contacting exhaust system
- Apply a light coating of an anti-seizing compound to the O 2 S threads. DO NOT allow the compound to enter sensor slots
- Tighten to: 37ft. lbs. (50 Nm)