Coolant has a protection level, and it drops
Coolant is not simply liquid in a tank. It is a mixture with a measurable freeze point, and that point moves over time.
Every time somebody tops a system up with water, the mixture weakens and the freeze protection falls. Do it a few times across a few years and a system that was good to well below zero is good to something a Montana night will find.
The test takes a moment and it is the single most useful thing that happens at a cooling system check. It tells you a number rather than an opinion, and the number is the one that matters in January.
Coolant also stops the system eating itself
The other job of coolant is corrosion protection, and the additives doing that work are consumed whether the vehicle is driven or not.
Depleted coolant looks fine. What it does is let the radiator, the heater core and the water pump corrode from the inside, so a cheap fluid change deferred long enough becomes a radiator, and sometimes a heater core, which is expensive out of all proportion to the part itself.
That is the argument for changing coolant on schedule rather than when something goes wrong.
The whole system, not just the fluid
A cooling system service covers the radiator, the pressure cap, the thermostat, the hoses, the water pump and the fan alongside the coolant.
A thermostat stuck open is the most under-diagnosed fault on the list. The vehicle never overheats, so nothing looks wrong, and the engine runs cooler than designed with worse economy and a cabin that never gets properly warm. Plenty of people treat that as a heater problem for years.
Hard work in both directions
Billings sits at the end of the range for this: hot work in summer, severe cold in winter, and a lot of vehicles here tow.
Towing is what finds a marginal cooling system. An engine pulling weight up a grade in July produces heat the system has to shed continuously, and anything slightly below par shows up there first. If you tow, have the system checked before the season rather than during it. See scheduled maintenance.
About this store
The Billings store is the Purcell location in Montana for vehicle repair, running sixteen services, managed by Larry Wagoner, open Monday to Friday, 7:30 AM to 5:00 PM.
See the national cooling system page for all 33 locations.