Oxygen Sensor Relay: Description and Operation

Oxygen Sensor Heater relay 5.9/8.0L
If 4 oxygen sensors are used, a separate heater relay is used to supply voltage to the sensors heating elements for only the 1/2 and 2/2 downstream sensors. Voltage for the other 2 sensor heating elements is supplied directly from the Powertrain Control Module (PCM) through a Pulse Width Module (PWM) method. Relay BoxApplications and ID.

Pulse Width Module (PWM) - 5.9L/8.OL: Voltage to the 02 sensor heating elements is supplied directly from the Powertrain Control Module (PCM) through two separate Pulse Width Module (PWM) low side drivers. PWM is used on both the upstream and downstream 02 sensors if equipped with a Federal Emissions Package, and only on the 2 upstream sensors (1/1 and 2/1) if equipped with a California Emissions Package. The main objective for a PWM driver is to avoid overheating of the 02 sensor heater element. With exhaust temperatures increasing with time and engine speed, it's not required to have a full-voltage duty-cycle on the 02 heater elements.

To avoid the large simultaneous current surge needed to operate all 4 sensors, power is delayed to the 2 downstream heater elements by the PCM for approximately 2 seconds.