These bi-annual safety and emission tests take about 90 minutes for a quick engineer to complete. Everything that is different from the original manufacturers type approval gets the car off the road, unless a particular mod has been tested, evaluated and approved by the authorities, and all the paperwork can be shown on the spot.
The guys doing the tests are all graduated engineers with about seven years university plus additional qualifications. Being a fully qualified mechanic is no way enough qualification to undertake those tests, so the official and strictly followed regulations. The bill for the "HU-test" is currently €124.- about US$ 160.- and must be paid by the owner. No valid test and the car is off the road.