B737-700 Captain reported a malfunctioning cargo fire test system during the preflight check led to the refusal of the aircraft despite the insistence by maintenance that the malfunction would clear itself in flight.

Narrative

I swapped into an arriving aircraft. Once onboard and doing the preflight; the Cargo Fire Test would not test on either side. From experience; I tried it again with the same results. ZZZ Maintenance was called and they said they would come out and look into it. A second or two later; before getting off frequency; Maintenance called us and said to stop pushing the cargo test button. A few minutes later the Mechanic came out and told us that they get this a lot and that the sensors get hot and will not test. It was 85 degrees. The Mechanic pushed the button about five times with the same result that I reported. I called Dispatch and he connected us with Person A in ZZZ1 Maintenance. Surprising to me he said the same thing that ZZZ Maintenance said that the sensors get hot and will not test properly; and that they will test in flight when they cool. I do not agree with their conclusion. The test does not say that and I have no guidance on that information. It does not say to push it several times until you get one good test to dispatch the aircraft. I refused the aircraft.