UNDERSTANDING YOUR SENSORS
When a sensor voltage falls out of specifications, the computer will illuminate a "CHECK ENGINE, SERVICE ENGINE SOON, OR MAINTAINANCE REQUIRED" light on the dash i.e the Malfunction Indicator Light(MIL). If this light is on in you car, it means the computer has received a bad reading from one of the sensors.
OXYGEN SENSOR: This measures the exhaust oxygen content. The computer fine tunes the mixture using data from this sensor.
FAILURE SYMPTOMS: Poor fuel economy, driveability, light on dash.
THE HEGO SENSOR (Heated Oxygen Sensor): This is just an Oxygen sensor, except it's HEATED! Heated Exhaust Gas Oxygen sensor. It is electrically heated so it works immediately on engine startup: the other type must be heated by the exhaust before it starts to work. Computers using these place a lot more emphasis on the data from the Oxygen sensor, and use this data sooner while the engine is still cold.
It is recomended to change this sensor on clocking 50,000 miles whether the MIL is on or not.