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My Car Temperature Gauge Reads Low

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Usually, when a car temperature gauge reads low, you can find the cause in one of three key areas:

  • Cooling system
  • Temperature gauge
  • Temperature gauge electrical circuit

Although not as dangerous as an overheating condition, engine over-cooling will accelerate parts wear and reduce fuel economy.

In most cases, the average car owner can diagnose and fix the problem using the vehicle's repair manual. Or using this guide, since you are dealing with a few components with easy access to on most vehicle models.

If you don't have the vehicle repair manual for your particular model yet, you can find a relatively inexpensive copy through Amazon.

Haynes manuals include:

  • Step-by-step procedures
  • Photographs and drawings
  • Parts location
  • Diagnostics
  • Electrical diagrams
  • Parts' specs
  • Maintenance schedule

And you can save a lot of money just by using your manual to keep up with your car maintenance to avoid unnecessary breakdowns.

The following sections describe the most common causes behind a car temperature gauge reading low.

If you want to dig deeper into one of the vehicle systems or components described in one of the following sections, you can consult your vehicle repair manual. Also, the Resources section at the bottom of this post lists a series of articles dealing with each potential problem.

In This Article

1. Coolant Level Low
2. Cooling Fan Runs All the Time
3. Thermostat Stuck Open
-- VIDEO
4. Testing an ECT Sensor
5. Failed Temperature Gauge
6. Resources

Warning!

Never remove the radiator cap on a hot engine. Extremely hot coolant may squirt out and seriously burn your skin.

1. Coolant Level Low

Visually inspect the coolant level at the radiator and the overflow tank. During the summer months, a cooling system in need of coolant will cause the engine to overheat. However, when the ambient temperature is cold, this same condition will cause a drop in engine temperature.

If the coolant level is low, you may want to check for coolant leaks. Check around the radiator, radiator hoses, engine block (around the head gasket area), heater core and heater hoses.

Consult your vehicle repair manual, and the Resources section at the bottom of this post as well.

Make sure the cooling fan works properly.

Make sure the cooling fan works properly.

2. Cooling Fan Runs All the Time

During normal operation, the cooling fan forces air through the radiator core and over the engine to help remove heat. Depending on your particular model, your vehicle is equipped with a clutch-driven fan or an electric-motor fan.

When the cooling fan gets stuck in the On position, it will run all the time, making it difficult for the engine to reach operating temperature.

Do the following quick test:

  1. Pop the hood open.
  2. Make sure the engine is cool.
  3. Set the transmission to Park (automatic) or Neutral (manual).
  4. Engage the Emergency brakes.
  5. Start the engine.
  6. Check whether the cooling fan is running. If the fan is running, this could be your problem. If you have a clutch-driven fan, the clutch is locked and needs to be replaced. On an electric fan, a shorted switch will cause this problem.

Consult your vehicle repair manual for the correct diagnostic procedure for your particular model. Also, the Resources section at the end of this post provides some help.

3. Thermostat Stuck Open

For the most part, a thermostat is a very reliable device that helps the engine run at the proper operating temperature.

After years of service, though, the thermostat may fail.

  • If the thermostat sticks closed, it will cause the engine to overheat.
  • The thermostat may fluctuate, causing the engine to overheat intermittently.
  • When it sticks open, it will cause the engine to operate below operating temperature.

If you suspect the thermostat is stuck open, do the following quick test:

  1. Pop the hood open.
  2. Make sure the engine is cool.
  3. Remove the radiator cap.
  4. Set the transmission to Neutral (manual) or Park (automatic).
  5. Engage the emergency brakes.
  6. Start the engine.
  7. Look through the radiator neck. If the coolant is flowing, the thermostat is stuck open. If the coolant doesn't flow until the engine reaches operating temperature (15 to 20 minutes), the thermostat is working properly.

Consult your vehicle repair manual, and the Resources section at the bottom of this post for more help on testing a car thermostat.

Also, the following video provides pointers about a failed thermostat that causes the temperature gauge to read low, and how to replace it.

Check the coolant temperature sensor.

Check the coolant temperature sensor.

4. Testing an ECT Sensor

Older vehicle models use a temperature sensor for the temperature gauge and another one for the engine computer. Newer models use a single engine coolant temperature sensor (ECT) for both.

If you own a scan tool and can read the ECT sensor, you can test this unit using this tool.

  1. Allow your engine to warm up for about 10 to 15 minutes.
  2. Turn off the engine and turn the ignition key to the On position, but don't start the engine.
  3. Connect your scan tool to the data link connector (DLC) of your vehicle. On most vehicles, this is located under the dashboard, on the driver's side (if necessary, consult your repair manual).
  4. Record the ECT temperature reading displayed on your tool.
  5. Pop the hood open, and measure the engine temperature using an infrared pyrometer or a contact-type temperature probe.
  6. Compare both readings. There shouldn't be a difference of more than 10F (5C) between the two. If the difference is greater than 10F, check the wiring and the connector to the ECT sensor for damage or corrosion. Also, check the ECT sensor using a digital multimeter.

4.1 Using a Digital Multimeter to Test an ECT Sensor

You can actually use a digital multimeter to test an engine coolant temperature (ECT) sensor. Consult your vehicle repair manual for this. Also, check the article Coolant Temperature Sensor Test listed in the Resources section at the bottom of this post.

5. Failed Temperature Gauge

When a temperature gauge fails, it may read high, low, fluctuate or stop working.

To quickly check gauge operation:

  1. Unplug the wire going to the temperature gauge sending unit, usually located on the engine. If necessary, consult your vehicle repair manual.
  2. Use a jump wire to ground the wire to the engine block.
  3. Turn the ignition key to the On position, but don't start the engine.
  4. Watch the temperature gauge. The temperature gauge will read hot if it is operating correctly. The gauge will read cold when you disconnect the jump wire from ground.
  5. If the gauge seems to operate properly when connected to and disconnected from ground, replace the sending unit.
  6. If the gauge fails to operate properly, check the gauge circuit and the gauge.

You can find more help in your vehicle repair manual, and the Resources section at the bottom of this post.

6. Resources

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2022 Dan Ferrell