Slow Down Light AFTER parking?

Don't replace cats. Try O2 sensor first.

Better still and I do not know the laws over there but why don't you punch the cats out and just have hollow cats then you should never have a slow down light.

What scanner do you use for reading codes?

I would like to have one of these to do the same?

 

I'm in Australia so emmissions is not a big issue for me hence the hollow cats.

For you Texas may be an issue.

I do not know how to test the thermocouples or O2 sensors but you could swap them from side to side and this may correct itself if the sensors are fine.

Alternatively you could buy a new thermocouple, O2 sensor and thermocouple station and at least you know they are brand new.

I replaced all these brand new a while back and have never had any issues since. I suppose as the car is 16 years old why not replace them brand new?

Just what I did but don't have to do on your end.

 

I have the same issue and I have been told its the cat ecu, swap the left and right to test them. the 2 sit near the wheel arch. Follow the cat wires. It's an easy process.

 

The CAT ECU (slow down system: SDECU) can fail. Probably the case here.

Its job is to protect the CATs from thermal destruction. It is a pain in the @$$ and costs $600 (a few years ago) to replace. It reads off the thermocouple (which rarely fails) and sends a signal to the engine ECU which turns on the light and a code. You an unplug it and make the error go away but don't do this unless you actually know the cat is not overheating.

Has nothing to do with either O2 sensor.

 

+1. The tip off is this happened after washing the car. The catalyst ECUs are prone to water instrusion as they age. The epoxy they use to seal them up (black on early versions, green on the latest updated ones) can shrink, and allow water, and dirt to enter causing failures.

There are several threads on this site on how to narrow down the bad one. Try a thread search.