Why Does My Car Overheat When The A/C is On?

  • June 24, 2025
car overheating with ac on car overheating with ac on

Summer, and all of its scorching hot days, have arrived. So, like any other day, you flip on the air conditioning to stave off the heat, but before long, your engine starts running hot. If your car overheats when the A/C is on, it’s not just annoying; it’s a red flag. While the A/C system is designed to keep you comfortable, it also adds extra strain to your engine. So, if something’s not right under the hood, that added load can push your engine past its limits. 

Whether it’s a failing fan, low coolant, or a blocked radiator, the connection between your A/C and engine cooling system is real. Below, we’ll break down why your engine overheats when the A/C is on, what signs to watch for, and how to fix the problem before it damages your engine. 

Warning Signs Your Car is Overheating with the A/C On

Overheating rarely happens without warning. Pay attention to these signs — they’re your chance to prevent engine damage caused by an overheating issue.

  • The Temperature Gauge Climbs: If you notice that your temperature gauge goes up when the air conditioner is on, it could simply be the engine reaching its operating temperature (190–225°F for most engines). But anything above operating temperature is a clear sign of stress on the engine cooling system.
  • Warning Messages/Lights: If any dashboard lights illuminate, or the message “A/C off due to high engine temperature” pops up on your car’s display, your car is likely conserving energy because your cooling system is struggling.
  • The Engine Overheats at Idle: If your car overheats when the A/C is on and the engine is idle, a lack of airflow could be the issue, indicating a failed radiator fan or another coolant system issue.
  • Cool Air Turns Warm: If the A/C stops blowing cold while the engine gets hotter, this might indicate a failure in either the air conditioning system, the cooling system, or both.

Why Your Car Runs Hot When the A/C Is On

When you turn on your car’s air conditioner, the A/C compressor kicks in and demands power from your engine. This places an additional load on the engine and its cooling components. Other than an increased demand for power, overheating has little to do with your A/C. It actually may not be your air conditioner causing the car to overheat. Instead, the same problems that would typically cause an engine to overheat are brought to light when the A/C kicks on. 

Ultimately, if your engine or cooling system is already experiencing issues, turning on the A/C can exacerbate them, potentially revealing a hidden problem. 

What Causes Overheating When the A/C Is On?

So, what are the common causes of engine overheating, especially when it seems like the air conditioner is causing your car to overheat?

1. Weak or Failing Cooling Fans

If your vehicle is equipped with electric radiator or A/C condenser fans, faulty wiring, failing motors, and other issues can cause their performance to drop. When this happens, there isn’t enough airflow to cool the engine, especially when idling. 

2. Low Coolant Levels

Low coolant is one of the most common (and easily fixable) reasons why cars overheat. The entire job of coolant is to absorb heat from the engine and disperse it through the radiator. Without enough coolant, your vehicle doesn’t stand a chance, especially during hot summer days. 

3. A Clogged Radiator

Rust, debris, and corrosion can block airflow and coolant flow through your radiator, blocking heat transfer and trapping it in the engine. While some amount of debris in the radiator is normal, filling your radiator with tap water instead of distilled or deionized water can cause excessive minerals and impurities to deposit inside, clogging passages and accelerating electrolysis.

4. A Failing Water Pump

Your water pump circulates coolant throughout the engine. A weak, failing, or clogged pump can slow or stop that flow, allowing temperatures to rise, especially under the added strain of the air conditioning system. 

What to Do About It

Whether your engine overheats when the A/C is on or off, you shouldn’t wait to address the issue. Driving with an overheated engine can cause severe engine damage, from blown head gaskets to overall failure. Fortunately, with a bit of troubleshooting, you may be able to prevent your car from overheating on your own, and if not, Firestone Complete Auto Care is here to help.

  • Inspect the engine bay and ground beneath your vehicle for cooling system leaks. If you find any, tighten the leaky connection or have faulty parts replaced by a professional.
  • Check your coolant level. If it’s low, top it off with a 50/50 mixture of antifreeze (which may be specific to your vehicle manufacturer) and distilled or deionized water.
  • Inspect your radiator for visible exterior damage or interior clogging. If you see junk in your radiator or it hasn’t been serviced recently, stop by Firestone Complete Auto Care for a radiator fluid exchange.
  • Watch your radiator fans by opening the hood and starting your vehicle. If your fans turn on once your engine reaches operating temperature, that's great! If they don’t, your fan is likely the issue.
  • Listen for squealing belts from a seized water pump or grinding/whining sounds from a failing one. 

You could spend time diagnosing the issue and seeing if your troubleshooting resolves it, but when getting stranded or experiencing overheating-related engine damage is at stake, our recommendation is to seek expert advice and/or repair. 

At Firestone Complete Auto Care, we offer more than just manufacturer-scheduled maintenance and oil changes. We also provide courtesy checks for no additional charge with every service, or complete vehicle inspections for those who want to ensure their vehicle is in top shape. In a complete vehicle inspection, we’ll examine your steering/suspension, exhaust system, brakes, and other systems to ensure they are in line with manufacturer specifications. 

Stay Cool With Firestone Complete Auto Care

So, if your car runs hot when the A/C is on, don’t wait for a breakdown — overheating can lead to costly engine damage. Stop by your local Firestone Complete Auto Care for professional diagnostics and repairs to keep your engine cool and your drives comfortable. Schedule an appointment today!

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