The temp gauge is a messenger. Shooting the messenger (thermostat, radiator, water pump — all of them) is how overheating gets expensive.

What it usually proves to be (in order)

1. 1Air trapped in the system after any coolant work — a bleed job, not a part
2. 2Thermostat stuck closed (overheat) or open (P0128, heat-up too slow)
3. 3A failing water pump impeller that still spins and still leaks no coolant
4. 4Fan/clutch or a collapsed lower hose restricting flow under load

What a parts-changer will tell you

Radiator replacement plus flush plus while-we-are-in-there package ($800–$1,800). It fixes some overheat cases and all of the shop revenue problems.

Typical guess-and-swap cost: $800–$1,800 (radiator + flush) when the fault was air, a $25 thermostat, or a fan clutch

The test that proves it

Infrared temp differentials across the radiator and block, upper vs. lower hose behavior, scan-tool coolant trace on warm-up, pressure test, then — only then — a head-gasket combustion test if everything else passes.