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

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Of course, the most typical reason because that an unusual analysis on her car's temperature gauge is a trouble with engine temperature. However, there are other reasons, because that example:

A negative temperature sending unit (the second most typical reason)A problem in the temperature gauge's electric circuitA fault in the control circuit native the car's computer (depending on car model)A bad temperature gauge

Any of these conditions may reason the temperature gauge to:

read lowread highgo up and down the scale

But her temperature gauge won't tell friend which reason you space dealing with. Unless you uncover some evident symptoms under the hood or the problem triggers the examine engine light, you'll should do a small diagnostic job-related to find out what's walk on.

The next sections describe the various ways her temperature gauge can provide you unexplained readings, and also the possible causes behind them. But first, let's take a short look at how the temperature gauge works, and also a couple of visible (and not so visible) symptoms the will aid you determine whether there's miscellaneous wrong v your gauge.


1. Just how a Temperature Gauge or Warning irradiate Works

2. Is it the Temperature Gauge or something Else?

3. Temperature Gauge reading Low

4. Temperature Gauge reading High

5. Temperature Gauge go Up and Down

6. Check the Temperature sending out Unit

7. Experimentation the Temperature Gauge


1. Just how a Temperature Gauge or Warning irradiate Works

Over the years the construction of the vehicle temperature gauge has not adjusted much. The operating values remain the same: A temperature sensor that reads engine temperature, and also a display to display the temperature.

Thus, the temperature gauge on your dashboard is your home window into the coolant temperature as watched by the temperature sending out unit. It allows you recognize when the engine is cool, at operation temperature, overheating, or fluctuating.

The sending unit is a thermistor. It is exposed top top one side to engine coolant. Its change resistance reaction to alters in coolant temperature:

When the engine is cool, as once the engine is off or has just to be started, the sending unit's resistance is at its highest, staying clear of electrical current flow. The gauge's indicator reflects this by continuing to be at the low finish of the analysis range. As engine coolant warms up, the sending out unit's resistance to reduce accordingly, and the gauge's indicator begins to rise. It'll stay at around the middle of the analysis scale, if the engine stays at operation temperature. A warning light won't come on uneven the engine overheats.

Until a few years ago, the temperature sending out unit sent out its information straight to the temperature gauge. However in most 1996 and newer models, the automobile computer is the interface in between the gauge or warning light and also the sending out unit.


A faulty radiator fan may reason the temperature gauge come fluctuate.

Photo courtesy the Tony Harrison on Flickr.

2. Finding out if the Temperature difficulty Is her Engine or her Gauge

Unfortunately, once the temperature gauge reads also low, as well high or fluctuates, girlfriend 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, that course.

Mostly, your temperature gauge will be accurate. But here's a quick diagnostic that can assist you as soon as you suspect something could be wrong v your gauge.

If possible, perform this initial inspection after the engine has been to run for much more than 20 minutes, or as quickly as her gauge suggests an overheating condition.

Pop the hood open and check the following:

The coolant level must be at the full Hot note on the coolant reservoir tank. A low level will cause engine overheating.

If your automobile doesn't have actually a reservoir or restore tank, wait because that the engine come cool and also check the level through the radiator neck. The level must be about 1/2 one inch below the bottom of the radiator neck but over the radiator core.Coolant need to look eco-friendly or orange. A rusty or cloudy shade indicates contamination and may cause overheating.The cooling fan have to start to run at roughly 230° F (110° C), which typically happens after 20 minute of engine operation. If the cooling fan doesn't revolve on, it will bring about overheating.

Compare the temperature that the engine head, close to the thermostat, and also the upper radiator hose. Both need to feel warm to the touch. The top radiator hose connects the top radiator tank to the engine in ~ the thermostat housing.

If the hose feels cool or warm however the engine feels hot, one of two people the thermostat is stuck closed, the radiator is clogged, or the water pump is no working. Any kind of of this will result in engine overheating.If the hose and also engine feel a bit warm but not hot, the thermostat is likely stuck open and also will protect against the engine from getting to operating temperature. The gauge will certainly read listed below the consistent temperature. Also, the heater might not produce enough heat.The reduced radiator hose must feel a little bit warm but not as warm as the upper hose. If both upper and lower radiator hose have around the same temperature, coolant is not circulating (overheating) or the thermostat is stuck open up (engine listed below operating temperature).Look around for potential indicators of leakage. Check radiator hoses, radiator, and water pump. If you view coolant leaking, the device is running the end of coolant and also the engine is overheating.

Compare her findings come the gauge reading and also decide whether the gauge analysis is accurate.

If girlfriend need an ext help, this other short article can help you uncover sources that engine overheating.

The complying with sections help you uncover potential troubles behind a gauge that is analysis low, high or fluctuating. Climate you'll find a procedure to test both the sending out unit and, if necessary, the temperature gauge.


A stuck-open thermostat will cause the temperature gauge to check out low.

Photo courtesy of Hoikka1 on Wikimedia.

3. If the Temperature Gauge reading Is Low

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

Potential troubles include:

A stuck-open thermostatLow coolant level (in a cold climate)Temperature gauge disconnected indigenous the temperature sending out unitBad temperature sending unitFaulty gaugeCircuit troubles in the gauge or sending unit (loose or corroded connectors).Car computer system issues, if the computer works together an interface


A clogged radiator is tough to diagnose since you can't watch coolant circulation restriction.

Photo courtesy that Gwydion M. Williams top top Flickr.

4. If the Temperature Gauge Is analysis High

A temperature gauge will typically rise above the middle variety when driving in heavy traffic, carrying or pulling a hefty load, or going increase a steep hill. However, if the gauge approaches the higher (hot) finish of the scale under common driving conditions, it typically points to difficulties with the cooling system.

Other potential reasons 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 instances this may cause detonation (a banging sound)

Loose radiator lid or faulty seals

Visually inspect radiator neck and cap for damage, and test cap pressure, if necessary.Coolant no flowing, because of a clogged radiator, partially open or stuck-closed thermostat, or a bad water pumpA radiator former blocked v bugs and also debrisBad coolant mixtureBad temperature gauge
A fail impeller in a water pump will stop coolant flow and also cause overheating.

Photo courtesy of Razor512 ~ above Flickr.

5. If the Temperature Gauge walk Up and Down

You may notification that her temperature gauge analysis goes up and down, fluctuates in some kind of pattern, or is erratic.

There might be several reasons for this:

Low coolant level delaying thermostat opening and also closing (check coolant level in the radiator).Air pockets in the cooling systemA coolant leakA partly clogged radiatorA water pump through a loose impellerA loose drive belt failing to run the water pump properlyA short, or loose or corroded connectors or wires, in the the electric circuit for the sending unit or gaugeCooling pan or relay problemsFaulty thermostatBad temperature gaugeBlown head gasket v exhaust gases messing up thermostat operation

Prevent computer system Damage!

If your auto computer acts as an interface between the sending unit and also temperature gauge or warning light, revolve the ignition move off once disconnecting wires indigenous the sending unit, warning light, or temperature gauge. You should do this to avoid induced-voltage damage to the computer. Above your automobile repair manual.

When you doubt a poor temperature gauge, most most likely the temperature sending out unit is the culprit.

Photo courtesy that Sfoskett~commonswiki top top Wikimedia.

6. Checking the Temperature sending out Unit

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

You can test the sending out unit's resistance or voltage making use of a digital multimeter (DMM).

Older auto models use a single temperature sending out unit connecting to the temperature gauge. Newer auto models connect to the electronic manage unit (ECU) or vehicle computer, which then sends information to the temperature gauge. Other car models, especially asian models, use 2 temperature sending units, one associated to the computer and the various other one to the temperature gauge.

You might need to consult the auto repair hands-on for your details model to locate components, determine wires and also terminal connections. If you don't have actually the fix manual, you deserve to buy a relatively inexpensive copy from Amazon. Haynes manuals come v step-by-step steps for many troubleshooting, repair and component replacement projects you deserve to do in ~ home.

Testing a sending out unit's resistance:

Locate the temperature sending unit. Commonly the unit is situated close to the thermostat housing, which connects to the upper radiator hose on the engine cylinder head.Unplug the sending unit electric connector.Measure the unit's resistance as soon as the engine is cold utilizing an ohmmeter. Resistance need to be high.Measure the unit's resistance once the engine is hot. Resistance need to be low.Compare your readings to specifications in your auto repair manual. If the sending unit specifications are correct, continue with the following steps.

Testing for reference voltage:

Turn the ignition move off.Unplug the wiring exploit from the temperature sending unit.Connect her voltmeter's red lead to the recommendation signal wire steel terminal.Connect your voltmeter's black cause the battery's an unfavorable (-) post.Turn the ignition switch to the top top position, however don't begin the engine.

You should have actually a 5 or 12 volts reference signal. If not, examine that cable for damage. If the wire is comes from the computer, and also the wire seems in good condition, the computer may be bad. Above your car repair manual.

Testing a sending unit's calculation voltage:

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

Connect the meter's red cause the sending unit's signal wire (consult your repair manual, if necessary).Connect the meter's black result in engine soil or the battery an adverse (-) post.With the engine cold, communicate the parking brake, start the engine, and also let the idle.Observe the voltage readings together the engine warms up. Her readings might be similar to the values in the following table, v a reference voltage the 5 volts to the sending unit from the computer system (some models may use a referral of 12 volts).

110F (43.3 C)


140F (60 C)


180F (82.2 C)


210F (98.8 C)


230F (110 C)


250F (121 C)


A negative temperature sending out unit can additionally cause a car's computer to make fuel mechanism adjustments based on incorrect data. You might see, because that example:

Black exhaust smokeHard starting on a warmed engineIncrease in emissions

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

7. Experimentation the Temperature Gauge

If the sending unit tested okay, and also you doubt a negative temperature gauge, you have the right to use this general procedure. You may need to consult your automobile repair hand-operated to identify or situate wires and also components.

Note: The procedure below requires grounding the wire the connects come the temperature sending unit. On part models, ground connection this wire can damage the temperature gauge. If necessary, above your vehicle repair hand-operated to uncover out if your car has this issue.

Note: If your vehicle computer controls her temperature gauge or warning light, follow the following three suggestions before proceeding with more troubleshooting steps:

If the inspect engine irradiate is on, check for diagnostic trouble codes (DTC) first. The password may point to the resource of the problem, generally a sensor or circuit issue.Make certain to turn the ignition switch Off before unplugging wires.Then revolve the ignition move On to examine gauge or warning light an answer as suggested in the following steps. This will prevent damages to the computer.

Continue Troubleshooting

Turn the ignition move Off.Unplug the sending out unit electrical connector and turn the ignition switch On. Whether the engine is warmed up or cool, the gauge have to read cold.

Ground the sending unit (signal) wire utilizing a jumper wire. Revolve the ignition move On. The gauge should read "hot" or the warning light need to come on.

If the temperature gauge responds together described, but not when linked to the sending out unit, the sending unit is bad.If the gauge reads higher than "cold" when you unplugged the sending unit, unplugged the cable at the temperature gauge. If the gauge currently reads "cold," the wire is shorted. If the gauge tho reads higher than "cold," replace the gauge.

If the gauge doesn't review "hot" once the wire is grounded, inspect the circuit fuse. If it is okay, soil the gauge terminal wherein the wire connects to.

If the gauge now reads "hot," inspect the cable for damage.If the gauge still doesn't review "hot," make certain there's voltage at the ignition terminal that the gauge. Follow the following procedure:Using a test light, affix its cable to ground and also the probe to the gauge's optimistic terminal. If the test light glows with the ignition turned on, change the gauge; otherwise, usage a jumper wire between the ignition switch and the gauge's confident terminal. If now the gauge functions as described over when unplugging and grounding the signal wire, then inspect the wire between the ignition and gauge; otherwise, change the gauge.

On part newer vehicle models, getting accessibility to a temperature gauge ~ above the tool cluster may be difficult. Sometimes, making use of a scan tool is the ideal option to troubleshoot this form of problem. Furthermore, on some models a poor temperature gauge calls for either calibrating the instrument swarm or installation a new cluster. Consult your auto repair manual.

On modern-day vehicles, you may use a scanner device to check the sending unit and also temperature gauge.

Photo in the public Domain.

If friend have identified that the temperature gauge has actually failed, you might want to inspect for a possible recall top top your car model ~ above this details issue.

A temperature gauge, or also an tool cluster, may come out of the manufacturing facility with concerns that might not manifest us after miles of operation.

You can examine for a recall on your automobile online, your local dealer or automobile repair shop.

This contents is accurate and also true to the finest of the author’s knowledge and also is not intended to substitute because that formal and individualized advice native a standard professional.

Questions & Answers

Question: ns am the owner of a Honda CRV Rd1. The temperature gauge reads high if driving end 60km/h and my AC unit stopped cooling. I pulled over and checked the both fans to be on and also the radiator and hoses didn't feel out of the ordinary. I even checked the overflow fluid and the water was cool sufficient to be touched. I'm totally confused. Any kind of ideas or suggestions?

Answer: There room some typical issues the usually can be traced come this trouble (but no the only ones): first, make sure the water pump is functioning properly. Check out if you have the right to hear any kind of noises (worn bearing) coming from the water pump. Second, a clogging radiator condenser (plugged fins-perhaps); third, concerns with a faulty AC compressor.

Question: which fuse is the temp gauge?

Answer: You can have a fused cable going to the instrument swarm if girlfriend don't have actually a different gauge. Watch under the lid for the fuse box.

Question: I have a 2005 Pontiac cool Prix and also the temperature gauge isn’t working. I’ve changed the battery, water pump, thermostat, and also I’ve had actually the computer reset. What is my trouble now?

Answer: inspect the gauge circuits, periodically it's a just a simple faulty floor or the gauge itself the is bad. Walk over the exam in the post. That should help to narrow under or find the issue.

Question: What does it typical when mine car's gauge registers only once the ignition is off?

Answer: inspect the temperature sending out unit and, if necessary, that circuit.

© 2019 Dan Ferrell


Dan Ferrell (author) on might 18, 2020:

Check the circuit. Make certain it's receiving referral voltage and also voltage calculation is coming out.

Francis on might 18, 2020:

I replaced my car ECU and also noticed the temperature guage stopped working. The doesn't move up or down. It's just iddle? please what can be the cause?

Dan Ferrell (author) ~ above November 25, 2019:

It's no safe to journey without a temperature gauge, yet you might want to try an aftermarket equipment in the meantime, so you can keep an eye on the temperature. Girlfriend don't need to make a long-term installation; just secure the properly and use the while you wait for the replacement. There space a variety of sites digital that can assist you cable the gauge, in case you require to. Simply google for 'wiring a car temperature gauge'. Hope this helps.

Guillermo Aveledo top top November 25, 2019:

My automechanic has done every the troubleshooting in differente sections of the cooling mechanism (radiator clogging, leaks, water pump, thermostat, cooling fan...) and also ruled lock out, assuring me that the problem is the Coolant Temperature Sensor.

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For a number of reasons, the part needed won't be easily accessible in my nation for a couple of months. The temperature gauge quiet goes Up and Down, erratically.