I am pleased to report that with a very small cash investment I was able to move a basic hand wired prototype to a home built PCB - let's call that version 0.8 - in a relatively short period of time. This unit has been tested on the car at idle and is proven to keep the light off. More work needs to be done yet, but PCB's have been ordered so that a working prototype - let's call that one version 0.9 - can be built and installed on the car for some real world testing. My car currently needs an MOT so the plan is to get the slow down light off permanently using the protoype, get the car MOT'd, and then do some on road driving to test it out more
And the most interesting part of all this is that the unit can be built for a LOT less than the cost of a new unit. It even fits inside the original case. I'll probably pot the thing as per the original but I want to expose some LEDs and a connector through the case so that's a bit more work. The hardware I have used also provides an interesting diagnostic capability because it can generate output that can be viewed on a computer, or even a mobile phone. So you can read the cat temps very easily and check that the ECU recognises presence of the thermocouple and see what output signal is being generated. I have even been able to code two test modes that will deliberately enable the slow down light and bank cut.
More details to come when version 0.9 has been built. Boards should be here in about 11 days.