How to approve the Gas Emission of our car: Approve ITV or RTV

Approving the emission of gases from our car is essential to pass the ITV (Vehicle Technical Inspection) or RTV (Vehicle Technical Review).
Generally high emissions of Hydrocarbons (HC), Carbon Monoxide (CO), high Oxygen (O2) are what prevent a car from passing the ITV or RTV.
Initially I show in the video the gas emission parameters for an idling engine and at 2500 RPM with HIGH EMISSIONS.
Then the engine is subjected to some maintenance and corrections:
Air filter change
Fuel filter change
Change engine spark plugs
Cleaning and maintenance of injectors
Inspection of vacuum and air leaks in hoses
Gas emissions are measured again and we notice a total reduction of Hydrocarbons and Carbon Monoxide to values below those required.
With this we easily approve the ITV or RTV.
If this were not the case, we have pending other tasks to be carried out in the engine:
Confirmation of operation of the oxygen sensor or lambda sensor
Engine Ignition Timing Recalibration
Finally if the above does not work: change the 3-way catalyst- HC, Co and NOx.