How To Test The Thermostat (1995-2000 1.6L Honda Civic)

By: Abraham Torres-Arredondo
Created: 24 November 2024
Updated: 26 November 2024
Articl Id: 1385

How To Test The Thermostat (1995, 1996, 1997, 1998, 1999, 2000 1.6L Honda Civic)

Testing the engine coolant thermostat on your 1995–2000 1.6L Honda Civic isn't difficult at all. In this tutorial, I'll walk you through the thermostat test using a scan tool with Live Data capabilities.

With your test results, you'll be able to quickly figure out whether the thermostat is functioning correctly or if it's causing your Civic's engine to overheat.

Contents of this tutorial:

Important Suggestions And Tips.
Symptoms Of A Bad Thermostat.
Checking The Thermostat's Performance.
Engine Thermostat Basics.
Other Tips And Suggestions.
More 1.6L Civic Tutorials.

APPLIES TO: This tutorial applies to the following vehicles:

1.6L Honda Civic CX: 1995, 1996, 1997, 1998, 1999, 2000.
1.6L Honda Civic DX: 1995, 1996, 1997, 1998, 1999, 2000.
1.6L Honda Civic EX: 1995, 1996, 1997, 1998, 1999, 2000.
1.6L Honda Civic LX: 1995, 1996, 1997, 1998, 1999, 2000.

Important Suggestions And Tips

TIP 1: To follow this tutorial, you'll need a scan tool with Live Data capability to monitor the engine coolant temperature. This method is the fastest, easiest, and safest way to check if the thermostat is faulty.

You don't need a Honda-specific or expensive professional-grade scan tool. A generic scan tool will work perfectly, and this tutorial is designed with that in mind.

TIP 2: Never open the radiator when the engine is warm or hot, whether it's running or not.

Opening a hot radiator can cause hot coolant to spray out under pressure, resulting in severe burns. Think safety all of the time and if you need to open the radiator cap, wait until the engine is completely cool.

TIP 3: Start your thermostat test with a completely cold engine.

TIP 4: The following tutorials will help you troubleshoot the source of an engine overheating problem (if the thermostat is OK):

Symptoms Of A Bad Thermostat

The thermostat in your Honda Civic's cooling system will fail in one of two ways:

Stays Stuck Open: This type of failure allows constant coolant circulation between the engine and radiator.
Stays Stuck Closed: This type of failure prevents coolant circulation between the engine and radiator.

When the thermostat is stuck open, you'll see:

Bad gas mileage.
The heater doesn't work (winter time).
The temperature gauge consistently shows a low reading or no reading at all.
Your Honda Civic fails the state emissions test.

On the other hand, when the thermostat is stuck closed, you'll see:

The engine overheats.
The temperature gauge hits its maximum sweep.
The temperature warning light illuminates to notify you of an engine meltdown.

Checking The Thermostat's Performance

Before we begin, here are two important reminders:

Start your thermostat test with a completely cold engine. This is crucial not only for your safety but also to ensure the test results are accurate.
Never remove the radiator cap once the engine starts warming up. Removing it can release pressurized hot coolant, which can cause serious burns.

Let's get started:

1
Check the coolant level in the radiator.

Top off the radiator with water or coolant if necessary.
2
Connect your scan tool and open its Live Data feature.
3
Check the temperature of the upper radiator hose with your hand.

NOTE: If the engine is cold (at ambient temperature), this hose will be cold. If the hose isn't cold, then you'll need to let the engine cool down further.
4
Check the temperature of the coolant on your scan tool.

On most scan tools this PID will simply say: Coolant °F or Coolant °C (see the photo in the image viewer above).

If you started out with an engine at ambient temperature, your scan tool should report a coolant temperature that should be within 10 degrees of ambient temperature. For example, If it's 95° F outside, then the coolant temperature sensor should report a temp reading between 95 to 105° F.
5
Start the engine and let it warm up.
6
At 150° Fahrenheit (65° C) on the scan tool, check the temperature of the upper radiator hose by lightly touching it with your hand.

What you're looking for is to make sure that the upper radiator hose is still cold. In other words, it should be the same temperature you noticed in Step 2, if so, continue to step 6.

If the hose is hot, this confirms beyond a shadow of a doubt, that the thermostat is either missing or stuck open. You can stop your testing here and replace the thermostat.
7
Let the engine continue running.
8
At 170° Fahrenheit (77° C) on your scan tool, lightly touch the upper radiator hose once again.

At this temperature point, the upper radiator hose should be hot to the touch.

If the upper radiator hose still feels cold at this point, don't worry about it just yet, continue to the next step.
9
At 200° F (93° C) on your scan tool, lightly touch the upper radiator hose once again.

The hose should now be hot -no ifs, ands, or buts.

Let's interpret your test results:

CASE 1: The radiator hose got hot at 170° F and stayed hot at 200° F. This confirms that the thermostat is functioning correctly.

Here's why: This is the normal and expected result of a properly operating thermostat since it confirms that the thermostat was closed below 170° F and that it did indeed open after 170° F to let the now hot coolant circulate to the radiator.

If your Civic is overheating, the following tutorials may be of help:

CASE 2: The radiator hose DID NOT get hot at 170° F nor at 200° F. This confirms that the thermostat is bad and is stuck closed. Replace the thermostat.

CASE 3: The radiator hose got hot below 150° F. This confirms that the thermostat is bad and is stuck open. Replace the thermostat.

Here's why: If the thermostat were working correctly, it would not let any coolant circulate within the hose and the hose would be at ambient temperature. Since the thermostat is stuck open (or missing) the coolant starts to circulate immediately and the hose will feel warm to hot as soon as the engine has been running for a few minutes.

Engine Thermostat Basics

If you've ever wondered why the engine has a thermostat and what it does, this section will clear things up (don't worry, I'll keep it simple).

The PCM (Powertrain Control Module) relies on the engine staying within a specific temperature range to efficiently manage emissions and optimize fuel economy. This ideal range is between 170° and 225° Fahrenheit (77° to 107° Celsius).

To maintain this range, the engine uses the coolant thermostat and the radiator fan motor, working together to regulate the temperature.

To be a bit more specific:

The thermostat's role is to prevent the engine from dropping below 170° Fahrenheit.
On the other end, the electric fan motor works to keep the engine temperature from exceeding 225° Fahrenheit.

The reason the engine needs to stay within this temperature range is simple: a cooler engine burns more gasoline to run smoothly, which leads to increased pollution.

However, once the engine warms up and reaches its optimal temperature, it requires less fuel to operate efficiently and produces fewer emissions.

In a nutshell, here's how they both keep the engine from overheating and running optimally:

When you start the engine, the thermostat is closed, preventing engine coolant from flowing to the radiator. This helps the engine warm up quickly.
As the coolant circulates solely within the engine, its temperature rises. Once it hits 170° F, the thermostat opens.
Once the thermostat opens, coolant starts circulating between the radiator and the engine.
Despite the coolant now circulating between the engine and radiator, its temperature will continue to rise.
When the coolant temp switch (radiator fan switch) senses 195° F, it activates the radiator fan relay to make the radiator fan turn ON.
The engaged fan now pulls more air across the radiator, cooling the coolant within it.
Since the thermostat is open, the cooler coolant in the radiator enters the engine to cool it down.
If the coolant temperature drops below 170° F, the thermostat closes preventing coolant flow to the radiator (to keep the engine within the specified range) and the cycle repeats itself.

The cool thing is that you can observe these temperature changes with your scan tool in Live Data mode and this is the way this tutorial shows you how to test the engine coolant thermostat.

Other Tips And Suggestions

Regular maintenance of your Honda Civic's cooling system is essential to saving money and avoiding major headaches down the road. While maintenance does take some time and effort, ignoring a small coolant leak or continuing to drive with an overheating engine can lead to costly repairs.

Unattended overheating or even minor coolant leaks often result in a blown head gasket —one of the most expensive and time-consuming problems to fix. That's why it's crucial to address these issues as soon as they arise.

Here are my personal recommendations:

If you notice cracks in the radiator's plastic tanks, replace or repair the radiator as soon as possible. These cracks can also prevent proper pressurization and, over time, may cause head gasket issues.
Flush the coolant at least every three years. From personal experience, I don't believe the claim that coolant can last 100,000 miles, and I suggest you don't either. If a full flush isn't possible, at least do a radiator drain and refill.
Replace the thermostat as part of regular maintenance, similar to replacing a fuel filter. While it doesn't need to be replaced yearly, doing it every three years is a good practice.
Replace the upper and lower radiator hoses (or any other coolant hose) if they're bulging at the clamps or the rubber feels too spongy.
If your vehicle overheats while driving, pull over to a safe location and have it towed to your home or a trusted repair shop.

More 1.6L Civic Tutorials

You can find a complete list of 1.6L Honda Civic tutorials in this index:

1.6L Honda Civic Index Of Articles.

Here's a small sample of the 1.6L Honda Civic tutorials you'll find in the index:

If this info saved the day, buy me a beer!

Applies To:

Honda Vehicles:

Civic DX 1.6L
- 1995, 1996, 1997, 1998, 1999, 2000
Civic EX 1.6L
- 1995, 1996, 1997, 1998, 1999, 2000
Civic LX 1.6L
- 1995, 1996, 1997, 1998, 1999, 2000