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Q: My car is overheating when driving instop and go traffic

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My car is overheating. It only overheats in stop and go traffic, and will cool down when driving at highway speeds. Here is a list of things that I have tried: 1. Changed three relays in the engine bay, labeled Fan No. 1, 2, and 3. 2. Removed the relay for Fan No. 1, and used wire to jump it. 3. Tested both fans, plugging them directly into the battery, they both work. 4. Shorted the thermostat at the bottom of the radiator. 5. Tested the resistance of the thermostat on the engine block. 6. Drained/refilled the coolant in the radiator. There are 2 fans on the radiator. I've read that one of them should turn on when the A/C turns on. This does not happen. The A/C is also making a grinding noise when it kicks on, but the air is still coming out cold. What else could I try?

Hi there, it sounds like you’re on the right path; it’s overheating because the fans aren’t turning on. We know your thermostat and water pump are both operating, otherwise it would most likely be overheating all the time. You said you replaced the relays, but what about the fuse? The power supply to the relays will be fused, and if that fuse is blown the fans will never turn on.

Check for power on both sides of the fuse with the key on. If the fuse is ok and you have power, check for power at the relay box. If you have power there and jumped the circuit with a wire before, and the fans still didn’t come on, then there is an open circuit somewhere, you just need to find it. If there is power at the relay and the fans work when you jump the relay, then you might have a failed engine coolant temperature sensor, which is what powers up the relay when the engine gets to a certain temperature. There could be another issue, though it’s hard to say without being able to see the car. To get a thorough, accurate diagnosis, I recommend contacting a professional to diagnose why your vehicle is overheating.

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