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I am in a weird situation. When I drive my car inside the city then the coolant in the reservoir is decreasing but when I am on highway then it remains same. For example yesterday I drove around 150 miles on highway buy it remains same. As I mentioned it is decreasing from the reservoir. There is no visible leak and there is no oil in coolant and vice verse (I recently got oil changes that is how I know it).

A side note; a few days back I got the following error on OBD2 scanner "coolant temperature below thermostat regulating temperature." My mechanic changed the thermostat (it was stuck open) but the coolant is still decreasing.

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    Welcome to Motor Vehicle Maintenance & Repair! I'm wondering if the coolant system wasn't bled correctly and it still has air in it?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Sep 12, 2023 at 21:26
  • @Pᴀᴜʟsᴛᴇʀ2 how can I check this?
    – Jalil Ahmad
    Commented Sep 12, 2023 at 21:32
  • No real way to check it. The best you can do is keep an eye on it and keep it full as needed. There will only be "so much" air in the system. If it is just air, at some point it will stop needing it. I'd think that should last more then a week to bleed out any which was left. I'm also assuming the level wasn't low (or lowering like it is now) prior to the thermostat replacement?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Sep 12, 2023 at 21:41
  • @Pᴀᴜʟsᴛᴇʀ2 it was lowering before that. I noticed it when I got this error on OBD2. Then I started checking it and it was decreasing. Then I got the thermostat replaced. After that I also changed the oil and filter.
    – Jalil Ahmad
    Commented Sep 12, 2023 at 21:46
  • If you are sure it's not leaking out, then you have to assume it's going elsewhere ... like out the exhaust pipe. If the oil was as expected and not chocolate milk shake, a head gasket leak would probably not be out of the question.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Sep 12, 2023 at 21:59

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Air pocket or blown head gasket if u don't see any leaks like you said. Coolent level goes up and down in the tank. that's what it's made for its not a Reservoir it's more of an expansion chamber... because of changing temperature and pressure in the cooling system. Back in the old days they just let it dump out on the ground and once in awhile you would refill the radiator.

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