99 355 slow down

The cat reacts to what comes in to it ,the more unburnt fuel that enters the cat the hotter the cat gets,the cat burns the up the unburnt fuel that enters it When a cat goes bad it dose so in one of two regular ways or a combination of the two.1 . the cat material inside becomes loose and starts to break apart and some times gets lodged inside ,this can cause more back pressure,but usually the stuff just starts to blow out the pipes.2.the reactive material that gets hot and burns off the gases will over tine loose it's punch and the cat will not get hot enough to burn the unburnt gases entering it .
Logic would point me to what ,is entering the cat ,to make it work hotter ,the cat is working you can see that buy the glow .I think exhaust gas readings at the manifolds would be wear I would start.my o2cents
DAVID

 

I have the same problem with the "slow down" light on my 96 355 Spider. I change out the cats to test pipes because of the unbearable heat in the engine compartment. My previous 348 does not have cats as it was a Euro model and cats was not a requirement. I ordered the test pipes and change them out. The test pipes has the threads for all the 3 Sensors on each side just like the Cats and they were all installed. Sure enough the heat was lot cooler in the Engine compartment. I did not feel any extra power. No " Slow down" light appear. Not until, I check with "Super Chip" and they told me they can improve on the ECU but I need to open up the ECU and give them the Number of the Chip.

I disconnect the battery and took the ECU out from behind a cover at the back of the drive seats (Right Hand Drive) open up the ECU box and recorded the Chip number. BTW it's not a plugged in Chip but soldered in. I put ECU back that night. the next morning I took the cars to work and after 10 miles the "Slow down" Light appears flashing intermediate. Engine still run normal, after 3 miles the "slow down" light complete lighted up and the engine lost more than 50% of it's power and I was struggling to get to the office.At traffic lights the idle speed drop to about 400rpm. It look like somebody just cut off 80% of the fuel or something. I finally make it to the office and just to check both banks are working, I check the tip of both exhaust and they are hot so both banks are working, just no power.
Since then, the "Slow down" comes on full time blinking and if its fully lighted again, I lost power. I tried last night, disconnected 2 of the 3 wires the wires on both side nearst to the rear of the test pipe, Shut of the battery and reset the car again. Start the car and the Blinking of the "Slow down" light continue, Drove over 40 mile yesterday, the Blinking of the "Slow down " will not go away but this time it has not fully lighted and I did not lose power. As long as it is blinking and not fully lighted, I'm okay but What can I do to eliminate it totally beside taking out the bulb? I cannot have a clogged Cats with test pipes. Do I need a DBII to reset the ECU?

 

William,

Once you put on test pipes - you will need to set the ECU with a ODBII to bypass the sensor (once you do this) you will not have any issues. Most independents will know what to do - due to the lack of Cats the sensor will pick up the unburnt gas and continue to make the light flash. I belive they just set it to bypass the reading altogether... Others on here might know specifically what you do in the ECU to correct this but I think this is the jist of it. It is a quick (cheap) thing to do - should take just a few minutes.