I own a 2001 Mercury Sable. The check engine light just came on.?

Assuming you have the 3.0L V6 then "they" (you'll need two. One per exhaust manifold) will cost you about $50 ea. As for putting them in it depends on where you go to get it done. They are about as easy to put in as a spark plug. You just need the right tool to do it. So if I had to guess, it's shouldn't cost you more then $175-$200 to get them both replaced.

BTW, an O2 sensor detects the oxygen levels in your exhaust and adjusts your fuel injectors to either add more or less fuel to the cylinders. This way your car isn't running rick (too much fuel for it to burn completely) or lean (not enough fuel which could cause pre-igintion).

First get the trouble code, and remember that this is Ford OBD-I system, so scan for codes in KOEO and KOER operations. The O2 sensor is cheap; it should be $45 for a Bosch plug-in type. An equivalent sensor for a Toyota can be $190 - 4 x as much! Then make sure to inspect the vacuum lines in the engine compartment, and replace any that are cracked or brittle. Inspect the lines that operate the EGR valve, and carry fuel vapor from the carbon canister to the engine.

A O2 sensor cost anywhere from 60-150.00 and labor should not be much it should not take any longer then 30-40 minutes.