1996 Mercury Marquis catalytic converter

Ok, First what codes were given? Your local Auto Zone will pull them for free.

Following is info on a cat.

When catalytic converters fail they normally clog up with debris and block the flow of exhaust gas from getting out of the system. This will cause tremendous performance problems. In extreme cases it will prevent the vehicle from starting at all. Most of the time it just limits engine performance by choking the flow through the engine. So how do you check a catalytic converter without removing it from the car? Sometimes an indication that a converter is clogged is that you don't go any faster when you push the gas pedal down. In addition there usually is a noticeable drop in fuel economy associated with a clogged catalytic converter. A totally clogged converter will cause the engine to die because of the increased backpressure.

There is no way for anyone to actually see a clog in a converter. Usually the only way to tell if a catalytic converter is clogged is to remove it and check the change in engine performance. When a mechanic suspects a clogged converter they may remove the O2 sensor from the exhaust pipe and see if there is a change in performance.

A catalytic converter relies on receiving the proper mix of exhaust gases at the proper temperature. Some engine oil additives or engine problems that cause the mixture or the temperature of the exhaust gases to change reduce the effectiveness and life of the catalytic converter. Leaded gasoline and the over-use of fuel additives can shorten the life of a catalytic converter considerably. Even some gasket sealers and cements can poison a converter.

A catalytic converter can also fail because of certain other factors. A number of problems could occur to the catalytic converter as the result of an engine that is out of tune. Any time an engine is operating outside proper specifications, unnecessary wear and damage may be caused to the catalytic converter as well as the engine itself. The damage is often the result of an incorrect air/fuel mixture, incorrect timing, or misfiring spark plugs. Any of these conditions could lead to a catalytic converter failure or worse.