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Q: Low coolant light comes on when AC, defrost or heater is on, but only if the car is in gear and stopped, what's wrong with my car?

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My low coolant light comes on when AC, defrost or heater (I will just say AC for short, the problem applies to all) is on, but only if the car is in gear and stopped. If I stop at a stoplight with the AC on the light comes on within 30 seconds, the light goes off as soon as the car begins moving again or if I turn the AC off. If I sit through the light with the AC on and the light lit up, at the next light the low coolant light will not come on. If the car is on and in park with the AC on the light doesn't come on. This happens in all weather and temperatures, at any time of day, whether the AC is on when I start the car or not, or if I let the car warm up before I turn it on or not.The AC, defrost and heater all work well, regardless of the low coolant light being on. I can't hear, feel or smell anything unusual, the engine coolant level is full, the temperature gauge reads normal temps.

My car has 107000 miles.
My car has an automatic transmission.

First, you want to make sure your cooling system is full. Check the radiator not the coolant jug. And check it when it is cold. If it needs coolant, add some. Of course, I don’t feel this problem is an actual low coolant condition. I would expect to find an electrical failure in the instrument cluster or from a component or wiring harness that is connected to the instrument cluster.

As a technician, I have access to reports from technicians across the country. I could not find any reports of this problem, so diagnosing this will require an experienced tech as this is not a common problem. In addition, this happens at intermittent times making it difficult to recreate. The most challenging part of this diagnosis will be recreating the symptom. I suspect it has something to do with the brake switch circuitry because it only seems to happen when you have your foot on the brake. I would begin my diagnosis there with a wiring diagram and test light in hand. The brake switch, instrument cluster, and environmental controls all interact with each other. Something is crossed either because of a failed module or damaged wiring in a harness.

Aside from making sure the coolant is full, this will not likely leave you on the side of the road. If you take it in to be diagnosed, be prepared for them to spend more time than usual to figure it out. Since it isn’t a key problem, you may need to live with it until it completely fails making it much easier to diagnose. Either way, I do recommend having a certified mechanic diagnose the warning light to make sure there isn’t anything serious going on.

Good luck.

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