Thanks again GVK. With all the info you have so generously provided already, I think I am gradually coming to understand how it all works. Originally I was wondering what turns the warning light on and how to prevent this happening if we have blanked the recirculation port. It now seems that to answer this question fully we need to understand what data gets fed from the various sensors to the PCM (ECU) and what the PCM then does with this data?

In what follows I am presenting my guess at the basic strategy which I am posting here for discussion/critique as I am by no means certain if this is correct. Apologies in advance for long-windedness but if interest and patience suffice, your views would be much appreciated.

In effect, it seems the system is fundamentally trying to keep the fuel to oxygen ratio in the cylinder to within certain limits as with high temperature and rich oxygen concentrations, more nox tends to form. Since the PCM knows how much fuel is flowing and it also knows the intake air (21% oxygen) massflow from the MAF, it has an idea of the oxygen/fuel ratio (OFR). By controlling the exhaust gas recirculation flow it can vary this OFR so it sends a signal to the EGR seeking to achieve this.

On simpler systems this would be basically a form of open loop control telling the EGR valve what position to move to. But on more sophisticated systems, an additional feedback signal is required to monitor how much exhaust gas is actually being recirculated. I suspect that this is what pin 3 (white and violet) is providing on the 3 pin plug. However, rather than an actual mass flowrate, as is produced by a MAF, this one is, in effect, a 'proxy' measure since it is actually the valve position that is being monitored.

On the motorised 5/6 pin plug something similar is presumably happening. EDIT. Incidentally I have now checked and my mate's 2.2 does have the motorised valve and it does have 5 terminals as GVK predicted.

If all this seems true so far, then we can start speculating about the EML. If the PCM sends an actuation signal to the EGR valve but it does not actually move (due to mechanical sticking, broken drive etc) clearly the anticipated position feedback signal will not correspond so it is logical that the fault light will come on. Furthermore, this will happen with either a 3 pin/vac EGR or a 5 pin motorised one. So if the EGR is faulty, we will get a light even if we blank the recirculation port.

The question now arises what happens if we insert a blanking plate on a system where the EGR valve is working freely? Since the position feedback signal to the PCM should now balance with the control/command signal from the PCM, then the PCM might 'think' everything is normal, so it will not put up a fault or corresponding warning light. However, if the PCW is 'cleverer' than this it might recognise that the OFR is probably still high, as there will have been no corresponding reduction in MAF output when the EGR opened (due to the presence of the blank). Hence it may still flag a fault on the system.

So I wonder which of these scenarios it is (if either)? Maybe only the system designers know for sure? I hope this makes sense and would welcome any further thoughts on this.