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Mazda Thermostat Troubleshooting

A thermostat is a valve that regulates the flow of coolant in an engine. When you start the car, the thermostat blocks coolant from reaching the engine until it heats up. Once the engine reaches the right temperature, the thermostat opens, allowing coolant to circulate freely. The thermostat opens and closes several times while the car is running. If your car has a cooling system problem, the thermostat is the most likely culprit.

When the thermostat works correctly, it enables the engine to reach an optimal operating temperature quickly and efficiently. It then keeps the engine running within an ideal temperature range. However, rust or contaminants can cause the thermostat to stick open or closed. When this happens, the engine either runs cold or overheats. Learning how to troubleshoot and repair your Mazda thermostat can prevent major damage to your motor and save you money.

Recognizing signs that your thermostat may be faulty

There are three main ways of discovering a faulty thermostat: error codes from the check engine light, your temperature gauge, and feeling the radiator hoses. If the thermostat is stuck open, your car will not run efficiently. If it is stuck closed, your vehicle will quickly overheat. Continuing to operate an overheated car or truck will permanently damage the motor. Keep an eye out for these thermostat warning signs to prevent engine damage.

Check engine light

It is a good idea to check the error codes on your vehicle when your check engine light comes on. You must have a code reader to be able to check the codes. You can purchase an OBD code reader for a reasonable price at most auto supply stores. There are two codes directly related to thermostat performance.

  • PO126 – This code indicates that either the coolant temperature sensor or the thermostat needs to be replaced. Most of the time, the thermostat is malfunctioning.
  • PO128 – This code indicates that the engine is not reaching the correct operating temperature within a set time after startup. This means the thermostat is stuck open, allowing coolant to circulate through the engine too soon.

Temperature gauge

Keep an eye on your temperature gauge. If your engine is running cold, the needle will stay on the low end of the spectrum. A cool engine is an indication that the thermostat may be stuck open, allowing coolant to circulate through the engine freely.

If the temperature gauge reaches the red range within about 15 minutes of driving your vehicle, the temperature gauge may be stuck closed. Do not continue to operate the vehicle; driving an overheated car can quickly ruin a good engine. Have the vehicle towed to your home or the nearest auto repair shop.

Radiator hoses

This method involves more confirmation than diagnosis, but it can help you pinpoint a bad thermostat. Make sure the engine is cold, then start your car. Place your hand on the top hose of the radiator. Keep your hands clear of any nearby fans or belts to prevent injury.

If the thermostat is working correctly, the hose should start out cool but warm up quickly. The thermostat is stuck open if the hose warms gradually. However, if the hose only heats up a little and the engine gets hot, the thermostat is stuck closed.

Known issues

There is a known issue with thermostats in Mazda 3 model years 2004-2010 and 2013. It affects models with 2.0 Liter, 4-cylinder engines. The check engine light comes on, and the error code indicates the thermostat is stuck open. The problem occurs at an average mileage of 92,000, but owners report the problem at anywhere between 8,800-165,000 miles on the motor. You must replace the current thermostat with a modified unit. The Powertrain Control Module (PCM) also requires a software update to fix this problem.

Replacing a faulty Mazda thermostat

The following steps are general guidelines for thermostat repair. Refer to your owner’s manual for exact vehicle specifications and part placement.

Gathering supplies

You will need several tools, including a socket wrench with 8, 10, and 12-millimeter sockets and three-inch, six-inch, and U-joint extensions. You will also need wrenches in the following sizes: 8, 12, and 14 millimeters. Finally, a pair of needle-nosed pliers and a drain pan are necessary.

Steps for removing the old thermostat

Mazda thermostats are often located on the side of the engine rather than the top. This can make it difficult to reach the parts from the top. You may need to raise the vehicle on ramps to reach everything and work safely. Follow these steps.

  1. Make sure the engine is cold. Working on a hot engine can cause severe burns.
  2. Remove the battery cover. Disconnect the negative battery cable
  3. Remove the undercover and the splash shield. They should come off as a single unit.
  4. Drain the engine coolant into the drain pan. If you are certain the fluid is good, you can reuse it by draining it into a clean container, then filtering it through muslin.
  5. Move parts out of the way. First, move the coolant drain, then remove the plug hole plate. Next, adjust the drive belt out of your way and remove the drive belt tensioner.
  6. Remove the next four parts in sequential order. First, detach the water hose, then the lower radiator hose. Next, remove the thermostat. Finally, remove the thermostat gasket. You may need a nail brush or piece of wood to scrape it off. Do not use a metal object to remove the gasket; it can damage the surface and prevent a good seal.

Testing the thermostat

You should verify the thermostat is the problem before replacing it. Sometimes you can see corrosion or buildup on the unit. In this case, go ahead and replace it. Depending on the extent of the corrosion, you may need to replace the thermostat housing as well. However, if the thermostat looks good, testing it can help you determine whether the problem is more extensive.

First, remove the thermostat from its housing. Heat a pot of water to the temperature indicated on the thermostat. Use a cooking thermometer to monitor the water temperature. Use a pair of needle-nosed pliers or a string tied to the thermostat to hold it in the water. For accuracy, make sure neither the thermostat nor the thermometer touches the sides or bottom of the pan.

Watch the thermostat until it opens. Remove and read the thermometer; it should be within 3-4 degrees of the temperature marked on the thermostat. If the temperature is outside this range or does not open completely, it is faulty. If the thermostat works correctly, it is not the issue. Replace the part and talk to a verified mechanic for further troubleshooting assistance.

Replacing the thermostat

 First, clean off any remaining residue from the old thermostat gasket. Replace it with a new one to ensure a tight seal. Install the parts in the reverse order of removal. Add engine coolant and inspect the system for leaks. It is also a good idea to verify that the new thermostat works by performing the radiator hose test.

Preventing coolant contamination

Coolant contamination frequently causes a thermostat to stick closed. Of the two thermostat issues, this is the one that can cause the most damage to your vehicle. Avoid these three contaminants to keep your thermostat working properly.

  • Mixing types of coolant – If you use different types of coolant at the same time, the formulas can interact and break down. This creates debris around the thermostat, keeping it from moving freely as the engine heats and cools.
  • Too much water – Some coolants must be diluted. However, if there is too much water in the system, it can cause rust. Oxidation around the thermostat also prevents it from opening.
  • Using stop-leak products in the radiator – It is tempting to find a quick and easy solution to radiator leaks, but using a stop-leak product will cost you more in the long run. The same ingredients that patch the leak also gum up the inner workings of the coolant system, including the thermostat.

Testing coolant

You can purchase a coolant tester at an auto supply store. In most cases, the tester sucks coolant into a measuring device. This tests the coolant’s ability to remain liquid when temperatures fall below freezing.

A digital multimeter can help you test for corrosion. With the engine cool, remove the radiator cap. Never open a radiator cap when the engine is hot; doing so can cause serious injury. Leave the cap off, then turn the engine on. Let it idle at 1500 revolutions per minute (RPMs) to warm up. Touch the negative probe to the negative battery terminal. Place the positive probe in the coolant, being careful not to touch the metal.

A reading of .4 volts or less indicates the coolant is good. If the reading is above .4 volts, it means the coolant components have broken down, allowing the liquid to transmit electrical impulses. Drain and replace the coolant.

Knowing when to replace your thermostat helps you keep your Mazda in top shape. Avoid contaminating the engine coolant. Test and replace the coolant regularly to keep your coolant system operating smoothly.

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