Q: How many O2 sensors are on a 2005 Cadillac CTS 3.6L V6 base sedan, I just purchased it used.

There are 4 oxygen sensors on your model, two upstream in the manifold area, and two after the catalytic converters (one per converter). Oxygen sensors are only replaced if the actual output wave form, and voltage range, is out of specification. If a sensor is claimed to be bad, you should be shown a graphical depiction of real time sensor operation and also be told to what extent the voltage is out of range. Without that information in hand sensor replacement is merely a guess and, in some case, the guess is wrong and thus after the sensor is replaced the trouble code remains simply because the code actually has multiple causes (including wiring issues and faults that have absolutely nothing to do with the sensor itself but rather other vehicles systems). On a mobile basis, replacement of the sensor(s) is economical and you can get an exact quote for your location (zip code) by referring to YourMechanic’s oxygen sensor replacement service page. Only those sensors that fail direct testing (i.e., direct observation of sensor outputs on a scope) need be replaced. If the sensor tests good, leave it alone. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.