Troubleshooting a Vehicle Cooling System

Car being towed

Car Being Towed | Source: Getty Images

Here's a common scenario: A driver sees the engine's “HOT” warning light activated on the dashboard, but bypasses a tow, continuing to drive home. Big mistake! Should your cooling system fail, the engine could overheat in as little as 10 minutes. So that act of driving the vehicle home to be spared the cost of a tow could actually wind up costing $3,000 or more, with the engine ultimately paying the price.

So what is the best way to prevent cooling-system failure? First, remember to change your coolant with regularity and replace your hoses as they age. Second, familiarize yourself with the rest of the major cooling system components. Here, we'll give you the warning signs of impending failure and let you know what your next steps should be.

Automotive Thermostat

An automotive thermostat is really just a valve to control the flow of coolant through the engine. It contains a spring, stainless-steel pin, rubber gasket, and a wax pellet. The wax inside the pellet expands as the coolant reaches 195-degrees Fahrenheit. The expanding wax pushes the stainless-steel pin against a solid retainer, forcing the pellet backwards just enough to open the valve. Once that automotive thermostat opens, the coolant flows and cools the engine. When the engine cools down, the wax contracts and the spring forces the automotive thermostat valve closed.

Voted Most Likely to Fail

Your car’s automotive thermostat is the part that fails most often, so it's the first place you should start if anything's amiss. Luckily, it’s also the least expensive part of the cooling system. Here’s how things can go wrong with your thermostat:

  1. When you use worn coolant, the formula loses it anti-corrosive properties.
  2. This leads to corrosion attacking all the metal parts in the engine and cooling system, including the stainless-steel pin.
  3. In turn, that corroded pin destroys the automotive thermostat’s rubber sealing gasket, allowing the wax to leak out.
  4. The automotive thermostat can then fail in either the open or closed position.


Stuck Open. In the “stuck open” position, the automotive thermostat prevents the engine from reaching full operating temperature. So if you notice that the temperature gauge on the dashboard never reaches the “normal” range, or the heater never expels hot air, suspect a “stuck open” automotive thermostat.

Stuck Closed. Conversely, a “stuck-closed” automotive thermostat can cause the engine to overheat quickly. So if you pull out of your driveway and barely cover any distance before seeing that “HOT” warning lighting up on your dash, suspect a “stuck closed” automotive thermostat.

Improper Temperature Rating. All automotive thermostats also have a temperature rating stamped onto the bottom of the wax pellet. Some drivers assume they can cure an overheating engine by either removing the automotive thermostat or installing an automotive thermostat with a lower temperature rating. That's not a good idea. A colder automotive thermostat won’t cure a cooling system’s underlying problem. In fact, it can lower gas mileage, cause poor acceleration, instigate misfires, even damage the catalytic converter. Always select the automotive thermostat with the temperature rating specified by your vehicle manufacturer.

Replace your thermostat each time you replace your coolant. We recommend investing in premium-brand automotive thermostats, like STANT thermostats. The price difference between an economy automotive thermostat and a premium thermostat is only a few bucks. Read a shop manual for proper installation.

Car radiator



To remove heat, the radiator fan blows outside air across thin fin tubes filled with hot coolant. If the fin tubes are clogged on the outside of the radiator, air can’t flow past them and the radiator fan can’t do its job to cool down the tubes.


Start troubleshooting by inspecting the radiator’s exterior. If you see bugs or dirt clogging the fins, hose down the radiator with a garden hose and spray nozzle.

If that doesn’t solve the overheating problem, check the condition of the radiator coolant. With the engine cold, remove the radiator cap and examine the coolant. If what you see after twisting off the radiator cap looks brown, muddy, rusty, or has debris floating in it, you’ve most likely got a clogged radiator.

Before replacing the radiator cap, you can try cleaning the radiator. You can do that with a radiator cleaning chemical and a cooling system flush. Put the radiator cap back on and see if the overheating is stopped. If that still doesn’t do the trick, no doubt the system is so corroded and clogged, the only option left is to replace the radiator.

Here's how to flush a radiator.


Coolant Temperature Sensor

Many late-model cars and trucks use electric radiator fans. To cycle the fans on and off, the computer relies on signals from the engine coolant temperature sensor mounted at the top of the engine. When the computer registers that the coolant temperature sensor reading has risen above a certain point, it turns on the radiator fans. However, if the engine coolant temperature sensor fails, the computer never turns on the radiator fans, leaving the engine to overheat.


How do you know whether the problem is the engine coolant temperature sensor or the electric radiator fan? Here’s a simple temperature sensor test.

  1. Start the vehicle and let it warm up a bit (but don’t let the dashboard temperature gauge go beyond “normal” range).
  2. Next, turn on the A/C and set it to “MAX” or “RECIRCULATE” mode.
  3. Now look at the radiator fans to see if they’re running:
    • If they are, that confirms the fan relay and radiator fans are, indeed, working properly.
    • If they don’t run, check the fuse. If that’s good, your problem may be a faulty engine coolant temperature sensor.
Car radiator fan

Car radiator fan


Radiator Fan

Radiator fans come in two varieties—mechanical and electric. Mechanical fans are powered by the car’s engine, and the fan itself rarely fails. But here’s the clincher: most mechanical fans are attached to a thermostatically controlled clutch mechanism, which can fail quite often.

If you have a mechanical fan and your engine overheats in city driving but is fine at highway speeds, you may have a failed clutch. Another sign of clutch failure: excessive fan noise, even when the vehicle is on idle. Clutches are easy to replace. Just remove the fan, the clutch retaining nuts, then swap in the new unit.


Electric radiator fans have a fairly low failure rate. But if the fan continually blows the fuse, it’s a safe guess that it’s on its way out. Replace the entire fan assembly. But first, consult your owner’s manual and follow the manufacturer’s recommendations to ensure warranties are not voided.

Water pump

Water pump

Water Pumps

Water pumps are pretty simple devices. With a drive pulley on one side, an impeller on the other, and a bearing/seal in the middle, you have all the makings of a water pump.


The water pump system can fail in one of three ways:

  • The water pump seal can fail, causing the water pump to leak. If you notice coolant leaking from your water pump, and it’s more than just an occasional droplet, you’ve got a water pump that’s about to fail. Replace the water pump as soon as possible.
  • The water pump impeller inside the water pump can fail. The plastic variety are especially prone to premature failure.
  • Air in the cooling system can cause a metal impeller to fail.

If you’ve got an overheating problem, and you’ve checked the radiator fan and radiator, plus replaced the thermostat, chances are you’ve got a failing water pump. Replace it ASAP. Choose from a new or rebuilt water pump, depending on the age of the vehicle and your budget. Be sure to also include a tube of RTV water pump sealer and fresh coolant to replace what you lose during the water pump repair.

Last updated December 15, 2020