How does an engine management system interface with PCM?
 

I'm almost ready to get an engine mangement system such as DFI or FAST and I have a burning question. How does the new system interface with the existing PCM? What happens to other functions the PCM does that do not have anything to do with the engine? What about those that are engine related?

I don't want suggestions on what to buy, I was only interested in what will and won't function when the systems are hooked up. Do a bunch of lights end up on all the time? Any help understanding how it works would be helpful.

Thanks

Take this for what it is, as I am not an expert in this area, but I have played with the stock PCM a bit. From my understanding, an aftermarket engine management system would completely replace the stock PCM, all the way. No SES lights would come one, as the PCM itself throws those codes, and if it were pulled out it couldn't display them. You may run into a problem if the car is 94+, as I don't think DFI can manage a transmission (assuming you have an A4), but other than that, which is a quess, I don't think any problems would really arise.