O2 Sensor (V6 And V8)

O2 Sensor Diagnostics

The Powertrain Control Module (PCM) performs diagnostics on the heater control, signal and sensor return circuits. Because of how the switching (4-wire) O2 Sensor operates, the circuit fault diagnostics for O2 Sensors are algorithm based. 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 the fault 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.

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 raw 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 differential signal voltage which is calculated using the raw signal voltage minus 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 raw 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 a Heater Control Circuit High or Low DTC and a Sensor Signal High DTC to be set together. 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 raw 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 degrees when compared to the other Switching (4-wire) O2 Sensors. As little as 2.0 Ohms can increase the temperature reading approximately 250 degrees on the scan tool. NOTE: When a normally operating Switching O2 Sensor is heated to operating temperature the typical duty cycle percentage will be 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 different.
O2 Sensor Signal Low Diagnosis:	Monitor the scan tool and start the engine. With an O2 Sensor Signal shorted to ground and the O2 Sensor cold, the raw signal voltage will be at 0 volts, but the (0-1) differential voltage signal will read -2.5 volts and increase toward 0 volts as the sensor heats up. When the sensor is warm, the raw signal voltage reading and the (0-1) differential voltage reading will both read near 0 volts. It will also pull the sensor return bias voltage low through the O2 Sensor. This will cause the raw voltage signals to switch between the 0-1 volt range on the other O2 Sensors. This may 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. As mentioned above, an open, short or any resistance in the heater circuitry can cause the signal voltage to remain high. Looking for abnormalities in the heater duty cycle or heater temperature of the affected sensor would indicate a heater control issue causing the signal circuit high fault. An open in a O2 sensor signal or return circuit will cause the raw signal voltage to read high, between 4.0 and 5.0 volts, and the (0-1) differential voltage reading to be near 2.5 volts for the sensor. If both of the Downstream O2 Sensors are 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 raw signal voltage readings for all O2 Sensors will read between 0-1 volts 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. However, this will cause the voltage reading on the sensor that has the shorted signal to read 0.0 volts on both the raw signal voltage and (0-1) differential 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 raw signal voltage. The (0-1) differential 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) differential voltage reading will start at 2.5 volts and decrease to 0.0 volts as the sensor warms up.