When I was cleaning out the boot just after I bought the car, I found this hand-written circuit diagram which I carefully put away and promptly forgot about. Pretty clearly at some point before the catastrophic engine failure, the engine swap to a 2009 was causing some grief - not surprising as the 2006 PCM expects an EGR valve but doesn't expect a variable valve timing cam phaser solenoid. Anyway, looks like whoever worked on the car has had trouble with the O2 sensors but didn't understand how they should work. Some cars use the PCM to *ground* the O2 heater circuit (so the PCM acts as a ground through an internal relay). Some cars use "pulse width" positive current *from* the PCM to drive the heaters, which the heaters being grounded elsewhere on the body. The 300C is the second type, although from the diagram it seems like the person doing the diagnostic thought they were the first type, have hooked up the heater to a positive power source and likely burned the pulse transistors in the 2006 PCM. I have ordered a 2nd hand 2009 PCM to check my theory. Likely this will require more wiring changes to eliminate the EGR altogether (currently it's hooked up electrically, but does nothing). Biggest issue here is that you need to program the PCM with your vehicle VIN so that it plays nice with the other computers in the system so that adventure will be interesting. AlfaOBD has the option to do the VIN programming so we will see how that goes.