Zephyr 2006, Fusion & Milan 2006-2007, MKZ 2007

Heated Oxygen Sensor & Catalyst Monitor Sensor



The Heated Oxygen Sensors (HO2S) perform the following:

Creates a voltage signal dependent on exhaust oxygen content.
Provides feedback information to the PCM used to calculate fuel delivery.

The catalyst monitor sensors:

Monitors oxygen content after it flows through the catalytic converter.
Provides voltage to the PCM used to calculate catalytic converter integrity.

Removal & Installation

  1. Disconnect the negative battery cable.
  3. If necessary, raise and safely support the vehicle.
  5. Disconnect the sensor electrical connector(s).
    If necessary, lubricate the sensor threads with penetrating and lock lubricant to make removal easier.

  7. Using Special Tool 303-476, or equivalent HO2S sensor socket, remove the sensor.

    Click image to see an enlarged view

    Fig. Use the special socket to remove the Heated Oxygen Sensor (HO2S) or Catalyst Monitor sensor


To install:

  1. Installation is the reverse of the removal procedure, noting the following:
    1. Apply a light coat of anti-seize lubricant to the threads of the sensor before installing it.
    3. Tighten the sensor to 48 Nm (35 lb-ft).



  1. Disconnect the Heated Oxygen Sensor (HO2S) connector. Measure the resistance between the HO2S heater and VPWR on sensor side of harness. The reading should be approximately 3-30 ohms. If resistance is not within specifications, the sensor may be faulty.
  3. With the engine running and HO2S sensor in place measure the voltage with digital multimeter between terminals HO2S and SIGRTN at the sensor harness. The voltage should read approximately 0.01-1.0V. If the reading is off or the voltage fluctuation is very slow suspect faulty sensor.
  5. Check for unmetered air leaks at intake manifold gasket leaks, hoses connecting to the mass air flow (MAF) sensor assembly, PCV system. Fuel calculations can be affected by unmetered air leaks.

    Click image to see an enlarged view

    Fig. Heated Oxygen Sensor (HO2S) connector-1: HO2S heater, 2: VPWR, 3: HO2S signal, 4: SIGRTN