my car failed emissions,

It is not possible to control an engine over normal EOBD/OBD2 even with a normal, modern, compliant car.

The protocol will only feed data to their diagnostic machine (like engine revs, or error status) but it won't rev the engine for them.

Thank you for clarifying that. It leaves me with a puzzle as to exactly what was meant.
As far as I’m aware to pass the test the rpm has to held steady for a specific time period and if that steady state is not held for the specific time the machine indicates a fail. It does not mean (as far as I understand it) that the emissions are unacceptable. It just means the test could not be completed.
So the puzzle is if the machine is not connected to the port in the car how does it know what the rpm is? It must connect somehow but perhaps not connect the MOT machine in such a way that it can “extract” everything it would like.
Id be interested to know what is involved if anyone can shed light on this.

Usual way to count rpms on emission test is by connection to car battery over battery harmonic content, measuring pulse count from an alternator.

ok, was away a few days, i am still waiting for the new cat and plug cables to arrive.  
i live in Belgium, they test the Co output by putting a sniffer in the exhaust , no other  connection is made, they ask to run at 2500rpm, that is all,  
i am not sure when i find time to ,have another try out.  if it passes i take the new cat back off and install the other cat again and save the new for future mot's..
i can ride to a local bmw service point and that guy has a handheld  co tester,   but i think that handheld tool will not be as sensitive as the machine the mot center uses? 
 

Re#212737 and counting rpm from alternator pulse.
Surely the alternator is belt driven and therefore rotating a speed determined by the pulley diameters that will potentially be at a different speed to the engine rpm although obviously related.
 

Replaced cat with a bosal. Replaced plugwires. Checked them with an ohm meter. Some where 5 kohm some 7 and one 9.2 kohm. All the new where 2.5 kohm. I also had replaced cap and rotor and plugs. It runs smooth, but it did run smooth before also.  Now i have to find time for another mot test.  But i am not realy in the mood at the moment because this takes another  free day. 5 hours waiting in a row untill it is your turn is normal in belgium.  But ill keep the forumguys informed when i went. 

You passed MOT now in Belgium? I have the same problem, also in Belgium...

Yes i passed. The cap and rotor looked bad. I put a bosal cat on new plugs and cables and it passed  . It took some time for the numbers to settle but it passed. 


next thing was getting plates and taxes...... paid 1280.  The government takes more than anyone on a car. From new untill now they take more than the total price for this car in taxes . Also on tires fuel and parts they earn money. Its crazy if you think about it how they rip you off.