How to Diagnose a Cooling System Problem

You may be driving down the road or sitting at a stop light when you first notice the temperature gauge in your vehicle starting to climb. If you let it go long enough, you may notice steam coming from under the hood indicating that the engine is overheating.

Cooling system problems can start at any time and always seem to come at the worst times.

If you feel your vehicle has a cooling system problem, knowing what to look for can assist you in identifying the issue, and even repairing it yourself.

Part 1 of 9: Understand your vehicle’s cooling system

The cooling system in your vehicle is designed to keep the engine at a consistent temperature. It keeps the engine from running too hot or too cold once it is warmed up.

The cooling system consists of several main components that each perform a different task. Each of the following components are essential to keep the engine running at the correct temperature.

coolant, thermostat, and water pump

second part of the graph

Part 2 of 9: Identify the problem

When your vehicle starts up fine when cold and if the temperature rises until it overheats and does not cool until the vehicle sits for a while, then there may be several different issues with your vehicle.

If any of the components fail, a series of issues can occur. Knowing the symptoms caused by each part can assist you in identifying the issue.

problem chart part 1

chart part 2

Part 3 of 9: Check for a failed thermostat

Materials Needed

A failing thermostat is the most common cause of overheating. If it is not opening and closing properly, then it must be replaced by a certified mechanic, such as one from YourMechanic.

Step 1: Warm up the engine. Start your vehicle and allow the engine to warm up.

radiator hose connected to a radiator with an arrow pointing to the hose

Step 2: Locate the radiator hoses. Open the hood and locate the upper and lower radiator hoses on the vehicle.

temperature gun being used

Step 3: Check the temperature of radiator hoses. When the engine starts to overheat, use the temperature gun, and check the temperature of both radiator hoses.

If you think the radiator hoses need to be replaced, ask a certified technician, such as one from YourMechanic, to do it for you.

Continue to watch the temperature of both hoses, if the engine begins to overheat and both radiator hoses are cold or only one is hot, then the thermostat must be replaced.

Part 4 of 9: Check for a clogged radiator

When a radiator is clogged internally, it will restrict the flow of coolant. If it clogs on the outside, it will restrict the airflow through the radiator and cause overheating.

Step 1: Allow the engine to cool. Park your car, allow the engine to cool and open the hood.

arrow pointing towards build up

Step 2: Inspect the inside of the radiator. Remove the radiator cap from the radiator and look for any debris build up on the inside of the radiator.

showing the outside of a radiator clogged with debris

Step 3: Check for external clogs. Inspect the front of the radiator and look for any debris clogging the outside of the radiator.

If the radiator is clogged internally, it must be replaced. If it is blocked on the outside, then you can usually clean it with compressed air or a garden hose.

Part 5 of 9: Check for a cooling system leak

A leak in the cooling system will cause the engine to overheat. Any leak needs to be repaired to prevent serious engine damage.

Materials Needed

Step 1: Allow the engine to cool. Park your vehicle and allow the engine to cool.

Step 2: Remove the cooling system pressure cap. Remove the pressure cap from the cooling system and keep it aside.

showing someone using a pressure tester

Step 3: Apply pressure. Using the cooling system pressure tester, follow manufacturer’s instructions and apply pressure to the cooling system.

  • Warning: The maximum pressure you should apply is the same pressure noted on the radiator cap.

Step 4: Check all the components for a leak. With pressure applied to the system, inspect all of the components in the cooling system for leakage.

Step 5: Add coolant dye to the system. If no leak is found with the pressure tester, remove the tester and add the coolant dye to the cooling system.

Step 6: Warm up the engine. Put back the radiator cap and start the engine.

Step 7: Check for a dye leak. Allow the engine to run for a while before checking for any signs of the dye, indicating a leak.

  • Tip: If the leak is slow enough, you may need to drive the vehicle for a few days prior to checking for signs of the dye.

Part 6 of 9: Check the cooling system pressure cap

Material Needed

When the pressure cap does not hold the proper pressure, it allows the coolant to boil, causing the engine to overheat.

Step 1: Allow the engine to cool. Park your vehicle and allow the engine to cool.

Step 2: Remove the cooling system pressure cap. Unscrew and remove the pressure cap from the cooling system and keep it aside.

showing a cooling system cap being tested

Step 3: Test the cap. Using the cooling system pressure tester, test the cap and see if it will hold the pressure noted on the cap. If it will not hold pressure, it must be replaced.

If you are not comfortable with pressure testing the radiator cap by yourself, contact a certified mechanic, such as one from YourMechanic to conduct the pressure test for you.

Part 7 of 9: Check for a faulty water pump

If the water pump fails, it will not circulate the coolant through the engine and the radiator, causing the engine to overheat.

Step 1: Allow the engine to cool. Park your vehicle and allow the engine to cool.

Step 2: Remove the cooling system pressure cap. Unscrew and remove the pressure cap from the cooling system and keep it aside.

Step 3: Check if the coolant is circulating. Start your engine. When the engine is warm, visually watch the coolant in the cooling system to see if it is circulating.

  • Tip: If the coolant is not circulating, then it may need a new water pump. The water pump test should only be done after you verify whether or not the thermostat is faulty.

water pump with dry tracks on it

Step 4: Visually inspect the water pump. A failing water pump sometimes show signs of leakage such as wetness or dry white or green tracks on it.

Part 8 of 9: Check if the radiator cooling fan is faulty

If the cooling fan is not functioning, the engine will overheat when the vehicle is not moving and if there is no airflow through the radiator.

Step 1: Locate the radiator cooling fan. Park your vehicle and apply the parking brake.

Open the hood and locate the radiator cooling fan. This can be an electric fan or a mechanical fan driven by the engine.

Step 2: Warm the engine. Start your vehicle and allow the engine to run until it begins to get hot.

Step 3: Check the cooling fan. When the engine starts to get above the normal operating temperature, watch the cooling fan. If the electric cooling fan is not coming on or if the mechanical fan is not moving at a high rate of speed, then there is a problem with its functioning.

If you have a mechanical fan that is not functioning, the fan clutch must be replaced. If you have an electric cooling fan, the circuit must be diagnosed before replacing the fan.

Part 9 of 9: Check for a faulty head gasket or internal problem

The most serious cooling system problems are internal engine issues. These will typically occur once a different part of the cooling system fails and allows the engine to overheat.

Materials Needed

Step 1: Let the engine cool. Park your vehicle and open the hood. Allow the engine to cool enough to remove the radiator cap.

person using a block tester

Step 2: Fit the block tester. With the radiator cap removed, install the block tester per manufacturer’s specifications.

Step 3: Observe the block tester. Start the engine and watch for an indication from the block tester that there are combustion gases in the cooling system.

If your test shows that there are combustion gases entering the cooling system, then the engine will need to be disassembled to identify the seriousness of the problem.

Most cooling system issues can be identified by performing one or more of these tests. Some issues will require further testing with other diagnostic tools.

Once you identify the faulty part, replace it as soon as possible. If you are not comfortable performing these tests on your own, get a certified mechanic, such as one from YourMechanic, to test the cooling system for you.


Next Step

Schedule Car is overheating Inspection

The most popular service booked by readers of this article is Car is overheating Inspection. Once the problem has been diagnosed, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. YourMechanic’s technicians bring the dealership to you by performing this job at your home or office 7-days a week between 7AM-9PM. We currently cover over 2,000 cities and have 100k+ 5-star reviews... LEARN MORE

SEE PRICING & SCHEDULING

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

Recent Car is overheating Inspection reviews

Excellent Rating

(1,277)

Rating Summary
1,169
54
13
10
31
1,169
54
13
10
31

Ruben

17 years of experience
113 reviews
Ruben
17 years of experience
Infiniti G35 - Car is overheating - Columbia, South Carolina
Ruben is very knowledgeable and on point. I am so glad he was the one chosen to come out to my vehicle. Also very prompt as well. Thanks, Shirley

Brett

25 years of experience
202 reviews
Brett
25 years of experience
Honda Civic - Car is overheating - Waxhaw, North Carolina
Arrived early and diagnosed car problem quickly. Very knowledgeable and explained everything. Even took the time to show me the issue and explain what needed to be done. I recommended 100 percent!!!

Rusty

23 years of experience
447 reviews
Rusty
23 years of experience
Honda Accord - Car is overheating - Arlington, Texas
My boy Rusty was able informed me of what was going on with my vehicle and it was much appreciated. Prayers and blessing to you and yours brother.

Jonathan

35 years of experience
432 reviews
Jonathan
35 years of experience
BMW Z4 - Car is overheating - Tampa, Florida
Will definitely recommend him to anyone. Did a great job, was quick and fair. Will use him again when the time comes.

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE

Related articles

How to Replace an Ambient Temperature Sensor
The ambient temperature sensor monitors the car's temperature inside and out. This sensor allows the AC to maintain comfortable cabin temperatures.
Symptoms of a Bad or Failing AC Condenser Fan
Common signs include lukewarm air coming from the AC vent, the car overheating while idle, and a burning smell in the car.
I Have No Idea What's Wrong With My Car - Where Do I Begin?
Any driver can use their senses to diagnose a car problem. Use your sense of smell, touch, and hearing to figure out what's causing the issue.

Related questions

Car losing power as driving

If you were driving and the engine started to overheat due to a stuck thermostat, then it would overheat in the first few minutes of running the engine. If you were driving and everything was okay and driving more than...

My car has started heating up, reached "cooling mode on" with temperatures of 250+, and it sounds like the fan stays on when I turn off car.

This can be caused by a number of things such as low coolant levels, a faulty thermostat, or a failing coolant fan switch. As you may know, the coolant fan switch helps to maintain the proper coolant temperature by turning...

Jessica Howe

Hi...yes, that is absolutely possible. If you would please review the slate of services (including all the various diagnostic services) available at this link, simply choose the service that best matches the difficulty you are having. Don't worry if it...

How can we help?

Our service team is available 7 days a week, Monday - Friday from 6 AM to 5 PM PST, Saturday - Sunday 7 AM - 4 PM PST.

1 (855) 347-2779 · hi@yourmechanic.com