Escort, XZ2 (2000-2003)

Heated Oxygen Sensor



A Heated Oxygen Sensor (HO2S) monitors the oxygen content in the three-way catalytic converter and transmits this information to the Powertrain Control Module (PCM). The PCM then adjusts the air/fuel ratio to provide a rich (more fuel) or lean (less fuel) condition.

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Fig. Inspect the oxygen sensor tip for all deposits

Perform a visual inspection on the sensor as follows:

  1. Remove the sensor from the exhaust.
  3. If the sensor tip has a black/sooty deposit, this may indicate a rich fuel mixture.
  5. If the sensor tip has a white gritty deposit, this may indicate an internal anti-freeze leak.
  7. If the sensor tip has a brown deposit, this could indicate oil consumption.
  9. All these contaminates can destroy the sensor, if the problem is not repaired the new sensor will also be damaged.
  11. Reinstall the sensor.
  13. Disconnect the Heated Oxygen Sensor (HO2S). Measure resistance between PWR and GND (heater) terminals of the sensor.
  15. With the engine hot-to-warm, the resistance should be 5-30 ohms.
  17. At room temperature, the resistance should be 2-5 ohms. If the readings are within specification, the sensor's heater element is okay.
  19. With the O2S connected and engine running, measure voltage with a DVOM by backprobing the SIG RTN wire of the oxygen sensor connector. The voltage readings at idle should stay below 1 volt. When the speed is increased or decreased, the voltage should fluctuate between 0 and 1 volt; if so, the sensor is okay.

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    Fig. Probe the terminals of the Heated Oxygen Sensor (HO2S) harness to test the heating element resistance