Why is My Car Temperature Gauge Not Working

Updated on June 6, 2019
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

 A temperature gauge may fail unexpectedly.
A temperature gauge may fail unexpectedly. | Source

Of course, the most common reason for an unusual reading on your car's temperature gauge is a problem with engine temperature. However, there are other reasons, for example:

  • A bad temperature sending unit (the second most common reason)
  • A problem in the temperature gauge's electrical circuit
  • A fault in the control circuit from the car's computer (depending on vehicle model)
  • A bad temperature gauge

Any of these conditions may cause the temperature gauge to:

  • read low
  • read high
  • go up and down the scale

But your temperature gauge won't tell you which cause you are dealing with. Unless you find some obvious symptoms under the hood or the problem triggers the check engine light, you'll need to do a little diagnostic work to find out what's going on.

The next sections describe the different ways your temperature gauge can give you unusual readings, and the possible causes behind them. But first, let's take a brief look at how the temperature gauge works, and a few visible (and not so visible) symptoms that will help you determine whether there's something wrong with your gauge.

Index
1. How a Temperature Gauge or Warning Light Works
2. Is it the Temperature Gauge or Something Else?
3. Temperature Gauge Reading Low
4. Temperature Gauge Reading High
5. Temperature Gauge Goes Up and Down
6. Checking the Temperature Sending Unit
7. Testing the Temperature Gauge
Some vehicle models include a warning light.
Some vehicle models include a warning light. | Source

1. How a Temperature Gauge or Warning Light Works

Over the years the configuration of the car temperature gauge has not changed much. Its operating principles remain the same: A temperature sensor that reads engine temperature, and a display to show the temperature.

Thus, the temperature gauge on your dashboard is your window into the coolant temperature as seen by the temperature sending unit. It lets you know when the engine is cool, at operating temperature, overheating, or fluctuating.

The sending unit is a thermistor. It is exposed on one side to engine coolant. Its variable resistance reacts to changes in coolant temperature:

  • When the engine is cool, as when the engine is off or has just been started, the sending unit's resistance is at its highest, preventing electrical current flow. The gauge's indicator shows this by remaining at the low end of the reading range.
  • As engine coolant warms up, the sending unit's resistance decreases accordingly, and the gauge's indicator begins to rise. It'll remain at around the middle of the reading scale, if the engine stays at operating temperature. A warning light won't come on unless the engine overheats.

Until a few years ago, the temperature sending unit sent its information directly to the temperature gauge. But in most 1996 and newer models, the car computer is the interface between the gauge or warning light and the sending unit.

A faulty radiator fan may cause the temperature gauge to fluctuate.
A faulty radiator fan may cause the temperature gauge to fluctuate. | Source

2. Finding Out if the Temperature Problem Is Your Engine or Your Gauge

Unfortunately, when the temperature gauge reads too low, too high or fluctuates, you don't know whether something is wrong with the engine, the gauge, or something else. Unless the gauge is in the red zone and you see steam coming out from under the hood, of course.

Mostly, your temperature gauge will be accurate. But here's a quick diagnostic that can help you when you suspect something might be wrong with your gauge.

If possible, do this initial inspection after the engine has been running for more than 20 minutes, or as soon as your gauge indicates an overheating condition.

Pop the hood open and check the following:

  • The coolant level should be at the Full Hot mark on the coolant reservoir tank. A low level will lead to engine overheating.

    • If your car doesn't have a reservoir or recovery tank, wait for the engine to cool and check the level through the radiator neck. The level should be about 1/2 an inch below the bottom of the radiator neck but above the radiator core.
  • Coolant should look green or orange. A rusty or cloudy color indicates contamination and may lead to overheating.

  • The cooling fan should start running at approximately 230° F (110° C), which usually happens after 20 minutes of engine operation. If the cooling fan doesn't turn on, it will lead to overheating.

  • Compare the temperature of the engine head, near the thermostat, and the upper radiator hose. Both should feel hot to the touch. The upper radiator hose connects the upper radiator tank to the engine at the thermostat housing.

    • If the hose feels cool or warm but the engine feels hot, either the thermostat is stuck closed, the radiator is clogged, or the water pump is not working. Andy of these will lead to engine overheating.
    • If the hose and engine feel a bit warm but not hot, the thermostat is likely stuck open and will prevent the engine from reaching operating temperature. The gauge will read below the regular temperature.
  • The lower radiator hose should feel a bit warm but not as hot as the upper hose. If both upper and lower radiator hose have about the same temperature, coolant is not circulating (overheating) or the thermostat is stuck open (engine below operating temperature).

  • Look around for potential signs of leakage. Check radiator hoses, radiator, and water pump. If you see coolant leaking, the system is running out of coolant and the engine is overheating.

Compare your findings to the gauge reading and decide whether the gauge reading is accurate.

If you need more help, this other post can help you find sources of engine overheating.

The following sections help you find potential problems behind a gauge that is reading low, high or fluctuating. Then you'll find a procedure to test both the sending unit and, if necessary, the temperature gauge.

A stuck-open thermostat will cause the temperature gauge to read low.
A stuck-open thermostat will cause the temperature gauge to read low. | Source

3. If the Temperature Gauge Reading Is Low

A temperature gauge that keeps reading low is not common unless there's a problem with the cooling system.

Potential problems include:

  • A stuck-open thermostat
  • Low coolant level (in a cold climate)
  • Temperature gauge disconnected from the temperature sending unit
  • Bad temperature sending unit
  • Faulty gauge
  • Circuit problems in the gauge or sending unit (loose or corroded connectors).
  • Car computer issues, if the computer works as an interface

A clogged radiator is hard to diagnose since you can't see coolant flow restriction.
A clogged radiator is hard to diagnose since you can't see coolant flow restriction. | Source

4. If the Temperature Gauge Is Reading High

A temperature gauge will normally rise above the middle range when driving in heavy traffic, carrying or pulling a heavy load, or going up a steep hill. However, if the gauge approaches the higher (hot) end of the scale under normal driving conditions, it usually points to problems with the cooling system.

Other potential causes include:

  • Low coolant level

    • Check both reservoir tank and radiator.
    • Check for leaks, including the radiator, water pump, heater core, hoses, and cylinder head gasket, as necessary.
    • In some cases this may lead to detonation (a banging sound)
  • Loose radiator cap or faulty seals

    • Visually check radiator neck and cap for damage, and test cap pressure, if necessary.
  • Coolant not flowing, due to a clogged radiator, partially open or stuck-closed thermostat, or a bad water pump

  • A faulty cooling fan

  • A radiator front blocked with bugs and debris

  • Bad coolant mixture

  • Bad temperature gauge

A failed impeller in a water pump will prevent coolant flow and cause overheating.
A failed impeller in a water pump will prevent coolant flow and cause overheating. | Source

5. If the Temperature Gauge Goes Up and Down

You may notice that your temperature gauge reading goes up and down, fluctuates in some kind of pattern, or is erratic.

There may be several reasons for this:

  • Low coolant level delaying thermostat opening and closing (check coolant level in the radiator).
  • Air pockets in the cooling system
  • A coolant leak
  • A partially clogged radiator
  • A water pump with a loose impeller
  • A loose drive belt failing to operate the water pump properly
  • A short, or loose or corroded connectors or wires, in the the electrical circuit for the sending unit or gauge
  • Cooling fan or relay problems
  • Faulty thermostat
  • Bad temperature gauge
  • Blown head gasket with exhaust gases messing up thermostat operation

Prevent Computer Damage!

If your car computer acts as an interface between the sending unit and temperature gauge or warning light, turn the ignition switch off when disconnecting wires from the sending unit, warning light, or temperature gauge. You need to do this to prevent induced-voltage damage to the computer. Consult your vehicle repair manual.

When you suspect a bad temperature gauge, most likely the temperature sending unit is the culprit.
When you suspect a bad temperature gauge, most likely the temperature sending unit is the culprit. | Source

6. Checking the Temperature Sending Unit

When you suspect that something is wrong with the temperature gauge, you want to first check the temperature sending unit, which is more accessible and more prone to failing.

You can test the sending unit's resistance or voltage using a digital multimeter (DMM).

Older vehicle models use a single temperature sending unit connecting to the temperature gauge. Newer vehicle models connect to the electronic control unit (ECU) or car computer, which then sends information to the temperature gauge. Other vehicle models, especially Asian models, use two temperature sending units, one connected to the computer and the other one to the temperature gauge.

You may need to consult the vehicle repair manual for your particular model to locate components, identify wires and terminal connections. If you don't have the repair manual, you can buy a relatively inexpensive copy from Amazon. Haynes manuals come with step-by-step procedures for many troubleshooting, repair and component replacement projects you can do at home.

Testing a sending unit's resistance:

  1. Locate the temperature sending unit. Usually the unit is located close to the thermostat housing, which connects to the upper radiator hose on the engine cylinder head.
  2. Unplug the sending unit electrical connector.
  3. Measure the unit's resistance when the engine is cold using an ohmmeter. Resistance should be high.
  4. Measure the unit's resistance when the engine is hot. Resistance should be low.
  5. Compare your readings to specifications in your vehicle repair manual. If the sending unit specifications are correct, continue with the next steps.

Testing for reference voltage:

  1. Turn the ignition switch off.
  2. Unplug the wiring harness from the temperature sending unit.
  3. Connect your voltmeter's red lead to the reference signal wire metal terminal.
  4. Connect your voltmeter's black lead to the battery's negative (-) post.
  5. Turn the ignition switch to the On position, but don't start the engine.

You should have a 5 or 12 volts reference signal. If not, check that wire for damage. If the wire is coming from the computer, and the wire seems in good condition, the computer may be bad. Consult your vehicle repair manual.

Testing a sending unit's output voltage:

On some models, you'll be able to back-probe the sending unit's signal wire using your voltmeter.

  1. Connect the meter's red lead to the sending unit's signal wire (consult your repair manual, if necessary).
  2. Connect the meter's black lead to engine ground or the battery negative (-) post.
  3. With the engine cold, engage the parking brake, start the engine, and let it idle.
  4. Observe the voltage readings as the engine warms up. Your readings may be similar to the values in the next table, with a reference voltage of 5 volts to the sending unit from the computer (some models may use a reference of 12 volts).

Temperature
Voltage
110F (43.3 C)
4.2
140F (60 C)
3.6
180F (82.2 C)
2.8
210F (98.8 C)
2.2
230F (110 C)
1.8
250F (121 C)
1.4

A bad temperature sending unit can also cause a car's computer to make fuel system adjustments based on incorrect data. You may see, for example:

  • Black exhaust smoke
  • Hard starting on a warmed engine
  • Increase in emissions

The following video gives you some visual clues about testing the sending unit.

7. Testing the Temperature Gauge

If the sending unit tested okay, and you suspect a bad temperature gauge, you can use this general procedure. You may need to consult your vehicle repair manual to identify or locate wires and components.

Note: The procedure below requires grounding the wire that connects to the temperature sending unit. On some models, grounding this wire can damage the temperature gauge. If necessary, consult your vehicle repair manual to find out if your car has this issue.

Note: If your car computer controls your temperature gauge or warning light, follow the next three suggestions before proceeding with further troubleshooting steps:

  1. If the check engine light is on, check for diagnostic trouble codes (DTC) first. The codes may point to the source of the problem, usually a sensor or circuit issue.
  2. Make sure to turn the ignition switch Off before unplugging wires.
  3. Then turn the ignition switch On to check gauge or warning light response as indicated in the following steps. This will prevent damage to the computer.

Continue Troubleshooting

  1. Turn the ignition switch Off.

  2. Unplug the sending unit electrical connector and turn the ignition switch On. Whether the engine is warmed up or cool, the gauge should read cold.

  3. Ground the sending unit (signal) wire using a jumper wire. Turn the ignition switch On. The gauge should read "hot" or the warning light should come on.

    • If the temperature gauge responds as described, but not when connected to the sending unit, the sending unit is bad.

    • If the gauge reads higher than "cold" when you unplugged the sending unit, unplugged the wire at the temperature gauge. If the gauge now reads "cold," the wire is shorted. If the gauge still reads higher than "cold," replace the gauge.

    • If the gauge doesn't read "hot" when the wire is grounded, check the circuit fuse. If it is okay, ground the gauge terminal where the wire connects to.

      • If the gauge now reads "hot," check the wire for damage.
      • If the gauge still doesn't read "hot," make sure there's voltage at the ignition terminal of the gauge. If there's voltage and the connection is okay, replace the gauge; otherwise, check the circuit between the gauge and the ignition switch or power source.

On some newer vehicle models, getting access to a temperature gauge on the instrument cluster may be difficult. Sometimes, using a scan tool is the best option to troubleshoot this type of problem. Furthermore, on some models a bad temperature gauge requires either calibrating the instrument cluster or installing a new cluster. Consult your vehicle repair manual.

On modern vehicles, you may use a scanner tool to test the sending unit and temperature gauge.
On modern vehicles, you may use a scanner tool to test the sending unit and temperature gauge. | Source

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

    © 2019 Dan Ferrell

    Comments

      0 of 8192 characters used
      Post Comment

      No comments yet.

      working

      This website uses cookies

      As a user in the EEA, your approval is needed on a few things. To provide a better website experience, axleaddict.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

      For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://axleaddict.com/privacy-policy#gdpr

      Show Details
      Necessary
      HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
      LoginThis is necessary to sign in to the HubPages Service.
      Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
      AkismetThis is used to detect comment spam. (Privacy Policy)
      HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
      HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
      Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
      CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
      Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
      Features
      Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
      Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
      Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
      Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
      Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
      VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
      PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
      Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
      MavenThis supports the Maven widget and search functionality. (Privacy Policy)
      Marketing
      Google AdSenseThis is an ad network. (Privacy Policy)
      Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
      Index ExchangeThis is an ad network. (Privacy Policy)
      SovrnThis is an ad network. (Privacy Policy)
      Facebook AdsThis is an ad network. (Privacy Policy)
      Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
      AppNexusThis is an ad network. (Privacy Policy)
      OpenxThis is an ad network. (Privacy Policy)
      Rubicon ProjectThis is an ad network. (Privacy Policy)
      TripleLiftThis is an ad network. (Privacy Policy)
      Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
      Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
      Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
      Statistics
      Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
      ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
      Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)
      ClickscoThis is a data management platform studying reader behavior (Privacy Policy)