O2 Sensor (4 CYL)

O2 Sensor Diagnostics

The Powertrain Control Module (PCM) performs diagnostics on the O2 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 have wide-band Upstream O2 Sensors and switching Downstream O2 Sensors, the Upstream and Downstream O2 Sensor Return circuits are separate circuits within the PCM. In this scenario, 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 circuit fault diagnostics on each of the wide-band Upstream O2 Sensors circuits. If there is an open or short in one of the wide-band O2 Sensors it should set a specific DTC. Do not use the readings for the wide-band Upstream O2 Sensor(s) in comparison to the Downstream O2 Sensor(s) when performing the diagnostics explained below.

With the scan tool in Data Display, filter on the O2 in the search function and select the following info:

Downstream O2 Sensor signal voltage - the raw signal voltage should switch between 2.5 and 3.5 volts on a normally functioning sensor.
Downstream 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.
Downstream O2 Sensor Heater driver operation - displays the heater duty cycle.
Downstream O2 Sensor 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 normalizes during that ignition cycle. Any of these conditions (open circuit or short to voltage) should set an O2 Sensor Heater Low or High DTC. Since the heater directly affects the output of the sensor signal, when the heater control is disabled, this will cause the affected O2 Sensor raw signal voltage to read high (usually 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 from a normally operating sensor. As little as 2.0 Ohms can increase the temperature reading approximately 250 degrees on the scan tool. NOTE: When a normal operating 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 1100°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 should 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 the Downstream 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 then 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 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.
P1621-O2 Sensor Reference (Return) Circuit Low Diagnosis:	The O2 Sensor Reference Circuit Low/High diagnostic is used to monitor the Downstream O2 Sensor Return circuit. If the return circuit is shorted to ground the raw signal voltage reading for the O2 Sensor 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. As mentioned in the O2 Sensor Signal Low Diagnosis above, a short to ground in the Downstream O2 Sensor signal circuit will also pull the sensor return low.
P1622-O2 Sensor Reference (Return) Circuit High Diagnosis:	A short to voltage in the downstream sensor return circuit will cause the DTC to set. The Downstream O2 Sensor 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 the O2 Sensor signal circuit 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.