Ford Repair: Location of O2 sensors, emission testing station, mercury marquis

Question
Could you please tell me the location of all four of the oxygen sensors on a
1997 Mercury Marquis. I have found the two that are opposite each other in
front of the catalytic converters but the other 2 elude me! I am not sure if there
are 4 but the person at the emission testing station insists that there are. After
replacing the two and clearing the codes I reset the "Check Engine" light. It
stayed off just long enough for the vehicle to get to the emissions testing
station and went back on. Thank you in advance for your timely reply.

Answer
First I would like to apologize for not getting back to you sooner. I had some families issues that required attention and was away from my computer for a few days. All post 1996 vehicles have an O2 sensor before and after the catalytic converter. The heater O2's located before the catalytic converters are usually located in the exhaust manifold itself. The other two sensors are located immediately after the catalytic converters. My guess is that those are the two that you have located. The sensors before the catalytic converters mounted into the exhaust manifold are very difficult to get to. You can probably handle the ones after the catalytic converter, but I would strongly suggest having a mechanic replace the pre-cat. sensors. While they are replacing the sensors, have them test the converters as well. With all four sensors throwing codes, have the codes pulled by a parts store. Most stores do this free of charge.