Q: Heat Gauge and Air Conditioning

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I have noticed when I run my A/C that my heat gauge will rise just a tiny bit. It doesn't get to 225 but does rise a very small amount when I am going uphill or stop at a light. It does go back down when I'm driving normally and even when it is reading above normal the coolant level remains the same in the reserve tank. Is this normal?

My car has 215000 miles.
My car has an automatic transmission.

Hi there. Although it is common for an engine temperature to rise slightly when the AC is on, 225 is a bit high. This might be an issue caused by two possible sources. First, it’s possible that your thermostat is not working correctly or opening later than it should. Second, it is also possible that you have an obstruction of coolant somewhere in the cooling system; perhaps in the heater core or air bubbles trapped in the system. Issues like this can be tricky to diagnose, so it might be a good idea to have a professional mobile mechanic come to your location and complete an overheating inspection; to help you isolate the issue and verify that there isn’t a major problem.

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