What do the O2 Sensors do? I've got 72,000 miles on my truck & I've been told they should be replaced. Also how many are there & how much do they cost?

 

Depending on the dealer you go through, they cost anywhere from about $55.00-$90.00 a piece and you likely have 4. There really isn't any need to replace them until they go bad though because once they do you'll know it. They usual symptoms are poor gas milage and engine performance from too rich of a fuel mixture.

Both the Haynes and Chilton manuals have a useful test procedure for determining if they are functioning properly. To test them all you'll need is a digital voltmeter and 2 safety pins for backprobing the connections.