In general the iTV station will "fail" the vehicle if the car is "non-standard". ie has spoilers, bodykit, bullbars, sidesteps etc etc. (Unless they were fitted as standard in the factory and are shown on the invoice)
Only Approved accessories, and by approved i mean as you say have homologation labels
attached and they have been fitted by an authorised supplier/fitter and the homologation certificate is present. The vehicle must have the "accessory" added to the Ficha Technica by an iTV station.
A very good example of this are towbars. They MUST be fitted by an approved dealer, The bar is supplied with a homologation certificate and a label will be attached to the bar showing the approval number. The car then has to be iTV'd and have the Bar added to the spec of the car on its Ficha. Otherwise a large fine will come your way if you are stopped by Traffico or the Guardia.
Now, when you take the car back for its next test and the label has fallen of the bar for example, the bar then becomes Unapproved. Your car will fail and a new bar will have to fitted..
If you buy a vehicle that has a bar fitted but has not been added to the Ficha then you have a problem. If the Homologation label is still attached but you do not have the original certificate, It cannot be added.
If you have the certificate but no label again no luck.
If you have the label and certificate there is a chance that you could "Persuade" the station to add it.
We are very very lucky here in the UK as we are not restricted to modifying our vehicles in any way we choose to.
Particularly those to the engine. Spain is a good example, even a "performance" airfilter is a no no.. In fact putting wheels/tyres that are bigger/smaller than those listed on the Ficha are Illegal!
There was some talk a few months ago in the press that the European Union were thinking about bringing the UK in line with the rest of Europe when it comes to Vehicle Modifications.
Lets hope that doesn't happen
