Ford Freestar, Monterey 2004-2006

Heated Oxygen Sensor



The HO2S detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected. A high concentration of oxygen (lean air/fuel ratio) in the exhaust produces a voltage signal less than 0.4 volt. A low concentration of oxygen (rich air/fuel ratio) produces a voltage signal greater than 0.6 volt. The HO2S provides feedback to the PCM indicating air/fuel ratio in order to achieve a near stoichiometric air/fuel ratio of 14.7:1 during closed loop engine operation. The HO2S generates a voltage between 0.0 and 1.1 volts.

Embedded with the sensing element is the HO2S heater. The heating element heats the sensor to a temperature of 1,472°F (800°C). At approximately 572°F (300°C) the engine can enter closed loop operation. The VPWR circuit supplies voltage to the heater. The PCM turns the heater on by providing the ground when the correct conditions occur. The heater allows the engine to enter closed loop operation sooner. The use of this heater requires the HO2S heater control to be duty cycled, to prevent damage to the heater.

Removal & Installation

  1. With the vehicle in NEUTRAL, position it on a hoist.
  3. Disconnect the negative battery cable.
  5. Disconnect the heated oxygen sensor (HO2S) electrical connector.
  7. Using the special tool, remove the HO2S sensor.
    Apply a light coat of anti-seize lubricant to the threads of the HO2S sensor prior to installation.

    Click image to see an enlarged view

    Fig. Removing the HO2S

  9. To install, reverse the removal procedure. Tighten the sensor to 34 ft. lbs. (46 Nm).