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Q: Can I add water or fluid to the reservoir till I can get a flush and fill?

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Heater blows cold air, radiator is full, reservoir is empty when car is cold. Temp gauge is normal, low-medium-high settings on blower work. It just doesn't heat.

My car has 80000 miles.
My car has an automatic transmission.

Yes, you can, and should, add coolant to the reservoir to make sure the engine cooling system does not run low. Add only a 50-50 (water-antifreeze ratio) coolant mix to the overflow tank, particularly if it is winter season. If you add water only, with no antifreeze, the plastic reservoir will crack if the water freezes. If the cooling system is full (despite the overflow reservoir being empty, that is you have no coolant "reserve"), a flush and fill will probably not resolve your no heat problem. Lack of cabin heat is typically due to blend door malfunctions, a faulty heater control valve, collapsed hoses and/or a plugged heater core. If the heater core is plugged it could very well require a more focused cleaning and possibly replacement. Prior to servicing the cooling system, you should resolve the no heat condition by requesting a heater diagnostic. The reason for that is if you end up having to replace the heater control valve, or heater core, you will have to drain the coolant out and of course, if you schedule a flush and fill before the repair, you’ll loose the coolant that you just paid for. Once the no cabin heat problem is resolved, a YourMechanic specialist can immediately perform a flush and fill right at your location.

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