Is Combustion Leak Tester Fluid result 100% accurate?

My 17 reg Q7 had cooling fan malfunction detected whist seen temperature gone up to 110 degrees after almost 15mins low speed drive (stuck in traffic) a burnt smell from bonnet I then stopped on hard shoulder waiting for recovery car, before I went to main dealer, my independent garage friend tested with Combustion tester fluid shown result that monoxide presented in cooling system. I then went to Audi dealer they fixed cooling fan but not committing/testing if there is indeed an engine oil seal leak/head gasket or not, and refuse accepting my friend's test result.

Does anyone can share some experience whether you used tester fluid before and proved it is an accurate way in testing