How To Test The Thermostat (Ford 1.9L, 2.0L)

When the thermostat goes BAD on your Ford 1.9L or 2.0L vehicle, it will either cause the car to overheat or, if it's winter time, the heater won't work.

In this article, I'll help you find out if the problem is due to the thermostat (either staying stuck closed or staying stuck open) without having to replace to see if it's good or BAD.

I've also included the test steps you'll need to see if the fan motor is coming on at the correct temperature.

To help you navigate this article, here are its main points:

  1. Important Suggestions And Tips.
  2. Symptoms Of A BAD Thermostat.
  3. Engine Thermostat Basics.
  4. TEST 1: Thermostat Operation.
  5. TEST 2: Fan Operation.
  6. Other Tips And Suggestions.
  7. More Test Articles.

Important Suggestions And Tips

TIP 1: The way I'm gonna' show you how to test the operation of the thermostat is by monitoring the temperature of the coolant with a scan tool (in Live Data mode). A simple code reader won't help, since a code reader does not have Live Data capability.

You don't need the Ford factory scan tool or an expensive professional technician level scan tool. A generic scan tool will do great and I've written this article with this tool in mind (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).

TIP 2: Never open (remove) the radiator cap while the engine is warming up or hot.

As the engine runs and warms up, the coolant inside the radiator, becomes hot and pressurized. So, opening (removing) the radiator cap, under these conditions, can cause you severe burns. Your safety is your responsibility... be careful and think safety all of the time.

TIP 3: Another very important thing, that is key to successfully troubleshooting the thermostat, is to start your test with a completely cold engine.

Symptoms Of A BAD Thermostat

Engine coolant thermostats are very predictable when they fail, since they fail in only one of two ways: they either stay stuck open or stay stuck closed. Here are some of the specific symptoms you'll see:

CASE 1: If the thermostat is stuck open, you'll see the following symptoms:

  1. BAD gas mileage.
  2. If it's winter time, the heater will not work.
  3. If your vehicle is equipped with a temperature gauge (and not just a temperature light), it will always show you a low reading or none at all.
  4. Won't pass the state emission's test.

Most folks are surprised to find out that the cooler the engine runs, the more fuel the vehicle consumes... since most folks intentionally remove the thermostat under the assumption that a cooler engine runs more efficiently.

CASE 2: If the thermostat is stuck closed, you'll see the following symptoms:

  1. The engine will overheat.
  2. Cooling fans come on, but the engine does not cool down.
  3. If your vehicle is equipped with a temperature gauge, it will max out.
  4. If your vehicle is equipped with a temperature light, it will shine nice and bright to let you know that an engine meltdown is occurring.

Engine Thermostat Basics

Knowing how the thermostat regulates the engine's temperature will help you to diagnose it quickly and efficiently. In this section I'm gonna' to briefly describe how the thermostat works...

In a nutshell, the thermostat's job is to help regulate the engine's temperature, so that your Ford's fuel injection computer can effectively control the emissions that the engine produces and to help you get the maximum amount of gas mileage possible.

The optimal temperature range that the thermostat (along with the radiator fan, coolant, water pump, etc.) try to keep your engine, is between 190° to 225° Fahrenheit (87° to 107° Celsius).

Why such a high temperature? Well, it's really not a high temperature... since this is the engine's normal operating temperature. It all boils down to the fact that the colder the engine is, the more fuel it needs to keep running smoothly (and unfortunately, at these temps, it pollutes more).

As the engine heats up and reaches the optimal temperature range described above... the engine needs less and less fuel to maintain it's optimal performance and of course it pollutes less.


Here are some more specifics:

  1. The thermostat is the one tasked with keeping the engine from going below 190° Fahrenheit.
  2. The fan motor (or fan motors) are tasked with keeping the engine below 227° Fahrenheit.
  3. When you start your vehicle, the thermostat is closed, thereby keeping the coolant from circulating to the radiator. This allows the engine to warm up faster.
  4. As the coolant circulates in the engine only, its temperature increases and when it reaches 190° F, the thermostat opens.
  5. With the thermostat now open, the coolant can now circulate between the radiator and the engine.
  6. Even tho' the thermostat has opened, the temperature of the coolant will continue to increase. When the coolant's temperature reaches 227° F, the fan or fans come on.
  7. The fan will bring down the temperature of the coolant (inside the radiator) which is circulating to the engine (due to the open thermostat). Once the temperature decreases down to around 200° F, the PCM then turns off the fan.
  8. If the coolant's temperature is brought down below 190° F, the thermostat will close. This helps keep the engine's temperature within the desired temperature range.
  9. With the fan off, the coolant now begins to absorb more heat and the process I've described above repeats itself continually the whole time the engine is running.

    The cool thing is that you can observe these temperature changes with your scan tool in Live Data mode and this is the way I'm gonna' show you how to test the thermostat.

Alright, let's get testing...