Theory Of Operation

O2 Sensor Heater Operation: For a typical Switching (4 Wire) Oxygen (O2) Sensor output signal to function the sense element must be heated to operating temperature. A resistive heater element is incorporated within the sensor to allow for rapid heating of the sensor to reach operating temperature faster and closed loop fuel control. The O2 Sensor Heater resistance is directly proportional to the heater temperature, meaning that as heater temperature increases, the heater resistance increases. The Powertrain Control Module (PCM) makes a calculation to determine the heater resistance and temperature. The heater resistance is calculated by measuring the Battery voltage and current draw on the heater control circuit, Then the heater temperature is determined using the resistance versus temperature characteristic. The heater temperature is monitored continuously and the heating rate is adjusted using Pulse Width Modulation (PWM) to prevent damaging the heating element. If the PCM detects a fault in the O2 Sensor heater circuitry the internal PCM driver is disabled during the current ignition cycle.

O2 Sensor Output Operation: Normal range of the O2 Sensor output is a 0 to 1.0 volt Analog to Digital (A/D) signal when the sensor is within the normal operating temperature range. The output voltage is generated by comparing the Oxygen content in the atmosphere, collected in a reference chamber to the Oxygen content in the exhaust stream collected in a comparison chamber. When the oxygen content is high (caused by a lean air/fuel mixture) the sensor produces a low voltage. When the oxygen content is low (caused by a rich air/fuel mixture) it produces a higher voltage. In some instances a negative offset output of up to -1.0 volt may be introduced if the sensor's reference chamber is contaminated. To allow for the negative voltage to be read, each O2 Sensor Return circuit has a 2.5 volt bias added to shift the signal voltage to between 2.5 volts and 3.5 volts.

O2 Sensor Diagnostics: The Powertrain Control Module (PCM) performs diagnostics on the heater control, signal and sensor return circuits. The circuit fault diagnostics for O2 Sensors are algorithm based because of how the O2 Sensor operates. This means an open or short in one of the O2 Sensor circuits can cause a fault to set against another of the O2 Sensor circuits. Understanding how to interpret the combination of DTCs and O2 Sensor data provided can aid in narrowing down where the fault is in the system.

On vehicle/engine combinations that use only the traditional switching O2 Sensors, both of the Upstream and Downstream O2 Sensor Return circuits originate from a single source within the PCM. Because of this, a short anywhere in the wiring in any of the O2 Sensor Return circuits will affect all of the Upstream and Downstream O2 Sensors.
On vehicle/engine combinations that have wide-band Upstream O2 Sensors and switching Downstream O2 Sensors, the Upstream and Downstream O2 Sensor Return circuits are separate within the PCM. In this case a short anywhere in the wiring in the Downstream O2 Sensor Return circuit(s) will only affect the Downstream O2 Sensor(s). The PCM performs individual diagnostics on the O2 Sensor Return circuit for each wide-band Upstream O2 Sensor.

With the scan tool in data display, filter on O2 in the search function and bring up the following info:

O2 Sensor signal voltage - the biased signal voltage should switch between 2.5 and 3.5 volts on a normally functioning sensor.
O2 Sensor (0-1) signal voltage - this is the raw O2 Sensor signal voltage without the 2.5 volt bias on the return circuit of a normally functioning O2 Sensor.
Heater driver operation - displays the heater duty cycle.
Heater temperature - this is determined by the total heater circuit resistance.
Closed loop fuel control state - displays as open/closed.

Using the Scan Tool for Diagnostics:
O2 Sensor Heater Diagnosis:	An open circuit or direct short to voltage or ground will cause the PCM to disable the heater PWM control. The heater will remain disabled for the remainder of that ignition cycle, even if the condition is intermittent and heals itself during that ignition cycle. Any of these conditions should set an O2 Sensor Heater Low or High DTC. Since the heater directly affects the output of sensor signal, and the heater control is disabled, this will cause the affected O2 Sensor biased signal voltage to read high (usually stays above 4.0 volts) and can also eventually set a Signal Circuit High DTC . It is not unusual to have both a Heater Control Circuit High or Low DTC and a Sensor Signal High DTC for the same sensor. Important Note: A small amount of resistance (3-4 Ohms) may not set a Heater Control Circuit DTC but will affect heater operation and cause the Sensor Signal High DTC to set. Monitoring the heater duty cycle and temperature for a sensor that is reading high on the biased signal voltage can help in diagnosing this issue. A heater circuit with 4.0 - 5.0 Ohms resistance can increase the heater temperature reading on the scan tool for the affected sensor by as much as 700°F when compared to the other Switching (4 wire) O2 Sensors. As little as 2.0 Ohms can increase the temperature reading approximately 250°F on the scan tool. NOTE: When a normally operating Switching O2 Sensor is heated to operating temperature the typical duty cycle percentage will range between approximately 30 and 50 percent. The typical heater temperature will range between approximately 1200°F and 1400°F on the scan tool. If there is an issue with the O2 Sensor heater or circuitry the PCM will disable the heater driver and the duty cycle will be 0 percent. Any issues in the heater circuitry, even a small amount of resistance, will cause the temperature reading to be noticeably higher than normal.
O2 Sensor Signal Low Diagnosis:	Monitoring the scan tool and starting the engine with an O2 Sensor Signal shorted to ground , and the O2 Sensor is cold, will show the biased signal voltage at 0 volts but the (0-1) raw voltage signal will read -2.5 volts and increase toward 0.0 volts as the sensor heats up. When the sensor is warm the biased signal voltage reading and the (0-1) raw voltage reading will both read near 0.0 volts. It will also pull the sensor return bias voltage low through the O2 Sensor. This will cause the biased voltage signals to read between the 0-1 volt range on the other O2 Sensors. This can also cause the O2 Sensor Reference (Return) Voltage Circuit Low DTC to set.
O2 Sensor Signal High Diagnosis:	The O2 Sensor Signal High diagnostic can fail due to several conditions. A open, short or any resistance in the heater circuitry can cause the signal voltage to remain high. Look for abnormalities in the heater duty cycle or heater temperature of the affected sensor to help indicate a possible heater control issue causing the signal circuit high fault. An open in a sensor return circuit will cause the biased signal voltage to read high, near 5.0 volts, and the (0-1) raw voltage reading to be near 2.5 volts for the affected sensor or sensors. An open in the signal circuit will have the same effect on the sensor. If the Upstream or Downstream O2 Sensors are both affected, it could indicate an open in the return circuit that is before the splice in the harness.
P1621-O2 Sensor Reference (Return) Circuit Low Diagnosis:	The O2 Sensor Reference Circuit Low/High diagnostic is used to monitor the O2 Sensor Return circuits. If any of the return circuits are shorted to ground the biased signal voltage readings for all O2 Sensors will switch between 0-1 volt instead of 2.5-3.5 volts. This happens because the bias voltage on the return circuit is pulled low for all sensors. As mentioned in the O2 Sensor Signal Low Diagnosis above, a short to ground in one of the sensor signal circuits will also pull the sensor return low for all sensors after the sensor warms up. However, this will cause the voltage reading on the sensor that has the shorted signal to read 0.0 volts on both the biased signal voltage and (0-1) raw voltage reading when the sensor is warm.
P1622-O2 Sensor Reference (Return) Circuit High Diagnosis:	A short to voltage in any of the sensor return circuits will cause the fault to set. The affected O2 Sensors will read 5.0 volts on the biased signal voltage. The (0-1) raw voltage reading will be 0.0 volts because the signal circuit voltage and return circuit voltage are both 5.0 volts. A short to voltage on one of the O2 Sensor signal circuits will cause the same fault condition and scan tool readings when the sensor is warm. If the condition is present when the engine is started and the sensor is cold, the (0-1) raw voltage reading will start at 2.5 volts and decrease to 0.0 volts as the sensor warms up.