What can happen if you do not replace a faulty temp sensor on a 2002 ford focus and how big of a job is it and how much does it cost?

If you dont replace a faulty O2 sensor you will not pass emissions, you probably are experiencing bad gas mileage as well. Sensor replacement is usually easy and straight forward. Somewhere in your exhaust pipe the sensor just merely screws into the exhaust. You will need a special O2 sensor socket readily available in any auto parts store for about 10 dollars. 02 Sensors usually run between 60-80 dollars but can run much higher in some cases(heated O2 sensors cost much more) I have a 1997 Explorer and found the O2 sensors in the manifold pipe under the drivers side of the vehicle. It was very easy to replace, I USED A VISE GRIP WRENCH, IT WAS VERY TIGHT AT FIRST, SO I LIGHTLY TAP THE VISE GRIP WITH A HAMMER, IT THEN CAME OUT EASYLY. THE SENSOR ( MOTORCRAFT ) COST ME $62.00 + TAX. - NICK L.