Unfortunately the response of your people sounds like a scatter gun effect with no real logic behind any of the things done to date. Most things in the control funtion do not fail and if they do then the fault code reader catches most of them. Catalysts tend only to fail if run to weak and then overheat - or someone jacks up the car on it.
There are several things that you can do before even thinking of changing anything:
Zetecs have a Schraeder valve on the fuel rail - check that you can see 2 bar pressure at idle. If not then you might have a blocked fuel filter OR, very unlikely, fuel pump problems. High pressure indicates a malfunction on the fuel rail controller or a blocked return line - someone jacked the car up on the return line? Low pressure leads to lean mixture and vice versa.
Check that all vacuum tubes from the manifold to the various sensors and breathers are intact and have not split. Any of these with aleak can lead to misleading problems. Also check the air bypass valve - ohmic check for continuity and the ability to move and seal. The crankcase breather valve if stuck open can cause a weak mixture but unlikely a rich mixture.
In one of the QM's I gave a list of all the likely voltages that you would expect to see on the various sensors for both MAP controlled engines and air meter controlled - I am away from home at present and do not carry such items on my web book. If you have a 'scope then you can check that you have a square wave output from the MAP sensor (if fitted) that is mark-spaced with varying pressure.
My experience is that at idle th Lambda sensor is not operative and that in general the idle governor and nothing else is in control. This does depend upon which controller and calibration you have got but completely throws the uninitiated, leading to the scatter gun effect noted above!!!
Hope that at least gives you something in which to get your teeth and stop haemoraging cash to someone who does not appear to know what they are doing.
Good luck!
Hamish
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