Kein problem, mein freund. Ich spreche Deutsch! But not that well. Here's what I now know. The oxygen sensor has nothing to do with the problem. It must be either one of the thermocouples, or one of the "black boxes", which are thermocouple amplifiers, also known as buffers. I have tried a few things: Disconnecting any one thermocouple (TC) gives a continuous flashing "slow down" light, so we can't do that. Disconnecting either buffer does the same thing. I disconnected one TC and substituted a K type thermocouple, and got the same result, so that won't work either. However, in the process of disconnecting and connecting all the TCs and buffers, the problem has gone away! At least temporarily. Now, as I understand it, a flashing light means slow down because the cat is getting hot. When it DOES get real hot, the light goes to a solid light, no longer flashing. The failure mode I have seen is that the light never flashes, just goes from off straight to solid, and almost immediately, half the engine cuts out ( bad news in traffic). I must assume that one of the TCs goes from OK to instantly very hot (unlikely), making the light go solid. If I see the problem again, I will buy one TC (about $150 - $200), and try it in each of the three places, until I hit the right one. Does this make sense?