Red Blinking Slowdown light

SDL light flashing at cold start up is most often 1 of 3 or all three of the following.

1. one or more cat ecu's are faulty or near dead
2. a thermo probe is not functioning properly
3. bad ground/open circuit on the engine ecu wire harness.

given that it is going off after warm up, I would think more 1 or 2.

 

Almost certainly a bad cat ECU - the potting has probably cracked. I fixed mine by sticking them in rice in the airing cupboard overnight to thoroughly dry, then resealing with high temp silicone sealant. That was two years ago - no problem since.

 

there's no damage being caused by a bad cat ecu.
so you are fine until you get get it fixed.
which should be sooner rather then later.

the risk of driving with a not properly functioning SDL system
is that if you do have a cat overheating you won't know.

but from what you discribed it seems one or more cat ecu's are acting intermittently
that's likely why it stops flashing eventually.

other then failing headers, cat ecu's are likely the second most discussed topic.
so it's very common.

 

Brian it is most likely one of six components of the SDL system. This system measures the exhaust gas temperature at three locations in your car aft of the cats (2) and after the bypass valve (1).via thermocouples (3)which send a mv signal to three individual voltage amplifiers/transmitters (Slow down ECU, CAT ECU, SDECU) which send the amplified signal to the main car ECU (Motronics) which reads that voltage signal and decides if your cats are to hot or not and then tells you via a SDL or a bank of cylinders gets shut down.

The usual culprits are sending units that the thermocouples plug into. One of 3. It could also be the thermocouples themselves. One of 3. You will sometimes get an OBD code that can help you figure out which side is at fault or you can start swapping known good parts around and see if it goes away or the more elegant way is to tap the outputs of the SDECUs and determine what voltage is being sent to the main ECU and therby see real time where the problem lies. YOu can then substitute/switch components and read the results of your efforts via your line taps.

You could also have a true overheated cat as well but if its happening on cold start that is probably not the case. If you aren't inclined to troubleshoot it yourself, someone knowledgeable about the above will be able to.