Owner's manual for a 2018 Honda Fit says to perform coolant service every 5 years or 50,000 miles.
But the car runs fine and has never overheated. Coolant levels in both the radiator and expansion tank are good, and I haven't had to top it off. Radiator cap is clean, still springy, and seals fine. The (original Honda) coolant is blue, transparent, and clean: no cloudiness, no oil, no rust, no debris, no signs of contamination. Hoses are tight, no cracks, no bulges, no signs of leaks, rubber is supple.
I got a pack of the Phoenix Systems Coolant Strips and tested the coolant from the top of the radiator. According to the most recent test strip, the pH is between 8 and 9 (i.e. still alkaline), and glycol% is between 50% and 60% (i.e. estimated freezing point below -34F, boiling point about 265F). I test every six months, and plan to drain/fill the coolant when the pH drops below 8 or glycol goes below 50%.
Is there any reason not to trust or rely on the coolant test strips? What other factors should be considered?