Ford Focus 2000-05

Oxygen Sensor



An Oxygen Sensor (O2S) is used on 1991-94 1.8L engines. The sensor is mounted in the exhaust manifold. The sensor protrudes into the exhaust stream and monitors the oxygen content of the exhaust gases. The difference between the oxygen content of the exhaust gases and that of the outside air generates a voltage signal to the PCM. The PCM monitors this voltage and, depending upon the value of the signal received, issues a command to adjust for a rich or a lean condition.


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 contaminants can destroy the sensor; if the problem is not repaired, the new sensor will also be damaged.
  11. Reinstall the sensor and disengage its electrical connection.
  13. Connect jumper wires from the sensor connector to the wiring harness. This permits the engine to operate normally while you check the engine.
  15. Start the engine and allow it to reach normal operating temperature. This will take around ten minutes.
  17. Engage the positive lead of a high impedance Digital Volt Ohmmeter (DVOM) to the sensor signal wire and the negative lead to a good known engine ground, such as the negative battery terminal.
  19. The voltage reading should fluctuate as the sensor detects varying levels of oxygen in the exhaust stream.
  21. If the sensor voltage does not fluctuate, the sensor may be defective or the fuel mixture could be extremely out of range.
  23. If the sensor reads above 550 millivolts constantly, the fuel mixture may be too lean or you could have an exhaust leak near the sensor.
  25. Under normal conditions, the sensor should fluctuate high and low. Prior to condemning the sensor, try forcing the system to have a rich fuel mixture by restricting the air intake, or a lean mixture by removing a vacuum line. If this causes the sensor to respond, look for problems in other areas of the system.