98 Grand Marquis rattle in exhaust system

Some of the problems....

Most common is a rusted heatshield or band clamp... the only way is to look and listen.....

Also possible.... broken catalyst monolith orruted baffle in the muffler.... again, there is a need to look and listen.

The internet is an aid - not a replacement....

Or one of the air tubes going into that cat is rotted off or broken, rattling? Someone please stop me if I am incorrect, but I think the oxygen IS the catalyst? Which if correct would lead me to believe not enough air is getting into the cat (insufficient catalyst)? Definately needs a visual inspection. I had to fix these years ago, forget if there were any codes involved.

The catalyst is a small amount of noble metal inside the converter; it promotes, but does not combine in the oxidation process. In the early days of emission controls Ford had external air delivered to the catalytic converters and a pump that provided it (called 'thermactor'). You will find that in my 1985 LTD, but it has been abandoned long before the

Sorry, Happy Traveler shot my theory full of holes for your car/year. Figures, since when I fixed this problem it was on a 1986 Ford Escort.

It sure seems suspicious that I get a rattle in the exhaust system when the engine is running and I get the code "insufficient catalyst". Is there an O2 sensor tht could be broken and flapping in the breeze?. This car has 2 cats and I hoping that I don't need t replace both. By the way, the engine runs great and gets good fuel economy, insufficinet catalyst or not. M

One of your catalysts has broken apart internally and the parts are rattling around inside. Smack each converter with a rubber mallet, you'll find out which one it is pretty quick. The fault code definition will tell you also. Bank 1 is the right side, bank 2 is the left side.

The defective catalytic converter has nothing to do with your engine's performance or fuel economy. It has to do with the air we all breathe. The code you see is from an 'open loop monitor' of the exhaust stream. The engine management computer does not use it for anything, except for letting you (and the emissions inspector perhaps) that something is amiss.