Simple. There are regulated tests that involve firstly a visual inspection, location, clearances, signs of distress, then there are Service inspection tests that confirm a free air path all the way from the outside air (For every Appliance) all the way to the mixing area, then there are gas service checks that involve inspecting the gas carrying components, upto the injector, then igntion devices, and clear flueway from the combustion area - These normally indicate the faults, then the test to confirm complete combustion, from is it getting the right amount of gas at the right pressure, and is it oxygenated correctly (ie flame pic or flue gas or both), Then if OF your flue performance tests. etc etc, all of these required inspections/tests then have their own sub headings to correctly identify the cause. if its installation then hey RIDDOR Time, If its old age, then new appliance, if its lack of service, then service it, if its worn parts, replace them. Isn't that what we get paid for.
If the appliance passes all your required tests and you have used your common sense, then it should be in service. Individual engineers will chose to either shut down immediately and wash their hands whilst more confident engineers will look for the cause and tackle it straight on. The list above just touches on the centre of your examinations, because there are a whole host of reaons why an appliance will not perform correctly, and 90% of the time evdience of this is visual. If you are professional you will offer the service you are paid for because you will be competent and confident in your call. remember the Unsafe Situations Manual is for advise only and is not gospel, you being the responsible person on site will use all your knowledge to address any unsafe situation to identify the cause and repatriation for a charge of course. This is why the Registration scheme is run on behalf of the HSE. The Badge means thats who you represent. Many appliances are put out of action daily because the engineer lacks the experience and knowhow to look further, or make a strong, firm decision based on actual evidence and facts. As an engineer i would never turn down a job, and one like this is always a challenge, WOW let me at it !, That's the feeling you want. You are being called for your expertise.
Good Luck Out There