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How to Diagnose a Broken Car Heater

parts of a car

Have you ever turned up your car heater in the winter to only notice that nothing happens? Or maybe you’ve noticed that when you try to defrost your windows, only colder air comes out of the vents! This may be due to a problem with your car’s heating system.

Here are a few ways to diagnose any issues in the radiator, thermostat, heater core, and other components that can make your heating system fail.

Method 1 of 4: Check proper fluid levels

Materials Needed

  • Gloves
  • Safety Glasses

  • Warning: Never do the following two steps while car is on or the engine is warm, it could lead to serious injuries. Always wear gloves and safety glasses for protection.

Step 1: Verify radiator coolant levels. Check the radiator fluid while the engine is cold - for example, before starting the car in the morning. Remove the coolant reservoir cap and verify that it is full. If it is low, this could be the reason why there is not enough heat being transferred inside.

check the coolant level in the reservoir and bottle

Step 2: Verify reservoir bottle fluid levels. The reservoir bottle holds the excess or the overflow of coolant from the radiator. Check if this bottle is full up to the “Max” line indicator.

The reservoir is usually a clear white oval or cylindrical shaped bottle which is next to or near the radiator. If its fluid level is low, it can also indicate that the radiator is low on fluid as well and that will cause poor heating conditions.

Method 2 of 4: Check the thermostat valve

Step 1: Turn on the engine. Start your car and switch on the heater.

Step 2: Check the temperature change on the dashboard. While the car is warming up in the mornings, always pay close attention to the cold and hot indicator on the dashboard.

If you notice that it is taking longer than usual to reach the point where the car is warm and ready to be driven, this could be an indicator of a stuck closed/open thermostat valve. This will also be a cause of poor heating in the cabin.

Method 3 of 4: Check the fan

Step 1: Locate the fan vents. Inside the dashboard, underneath most glove compartments there is a small fan which circulates the warm air in the cabin area.

Step 2: Check for a broken or faulty fuse. If you don't feel the air coming through the vents, it could be related to the fan not working. Refer to the owner's manual of your car to find the fuse box location and locate the fan fuse. Check the fuse, if it is still functional, the issue may be due to a faulty fan.

Method 4 of 4: Check for a faulty heater core

Step 1: Check for a clogged heater core. This component of the heating system is a smaller radiator located inside the car compartment underneath the dashboard. Warm coolant flows inside the heater core and when the heater is turned on, it transmits the heat to the cabin area.

When the heater core is clogged or dirty, the coolant flow is insufficient and that can diminish the heat inside the car.

Step 2: Check for a leaking heater core. Check the floor mats in the cabin and see if they are damp or if they have the smell of coolant.

When the heater core gets damaged, it will become very noticeable because the cabin area on the floor mats starts getting wet and the smell of coolant is present. This also leads to poor heating conditions.

  • Tip: Ensure that you check the air conditioning before those hot days of summer as well.

A heating system that functions properly is an important part of your car. Also, a broken car heater will adversely affect the performance of your car’s defroster, which in turn hinders visibility, and limits your ability to drive safely. If you notice any issues with your car’s heater, ensure that you perform a thorough check of the system and get any problems fixed at the earliest.

If you are not comfortable with performing this process on your own, get a certified professional, such as one from YourMechanic, to perform the heater inspection for you.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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