Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.
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.
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
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.
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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. Any 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. Also, the heater might not produce enough heat.
- 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.
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
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
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.
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:
- 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.
- Unplug the sending unit electrical connector.
- Measure the unit's resistance when the engine is cold using an ohmmeter. Resistance should be high.
- Measure the unit's resistance when the engine is hot. Resistance should be low.
- 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:
- Turn the ignition switch off.
- Unplug the wiring harness from the temperature sending unit.
- Connect your voltmeter's red lead to the reference signal wire metal terminal.
- Connect your voltmeter's black lead to the battery's negative (-) post.
- 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.
- Connect the meter's red lead to the sending unit's signal wire (consult your repair manual, if necessary).
- Connect the meter's black lead to engine ground or the battery negative (-) post.
- With the engine cold, engage the parking brake, start the engine, and let it idle.
- 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).
110F (43.3 C)
140F (60 C)
180F (82.2 C)
210F (98.8 C)
230F (110 C)
250F (121 C)
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:
- 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.
- Make sure to turn the ignition switch Off before unplugging wires.
- 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.
- Turn the ignition switch Off.
- 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.
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. Follow the next procedure:
- Using a test light, connect its wire to ground and the probe to the gauge's positive terminal. If the test light glows with the ignition turned on, replace the gauge; otherwise, use a jumper wire between the ignition switch and the gauge's positive terminal. If now the gauge works as described above when unplugging and grounding the signal wire, then check the wire between the ignition and gauge; otherwise, replace the gauge.
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.
If you have determined that the temperature gauge has failed, you may want to check for a possible recall on your vehicle model on this particular issue.
A temperature gauge, or even an instrument cluster, may come out of the factory with issues that may not manifest themselves after miles of operation.
You can check for a recall on your vehicle online, your local dealer or car repair shop.
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
Question: Which fuse is the temp gauge?
Answer: You might have a fused wire going to the instrument cluster if you don't have a separate gauge. Look under the lid for the fuse box.
Question: I am the owner of a Honda CRV Rd1. The temperature gauge reads high while driving over 60km/h and my AC unit stopped cooling. I pulled over and checked that both fans were on and the radiator and hoses didn't feel out of the ordinary. I even checked the overflow fluid and the water was cool enough to be touched. I'm totally confused. Any ideas or suggestions?
Answer: There are some common issues that usually can be traced to this problem (but not the only ones): first, make sure the water pump is working properly. See if you can hear any noises (worn bearing) coming from the water pump. Second, a clogging radiator condenser (plugged fins-perhaps); third, issues with a faulty AC compressor.
Question: I have a 2005 Pontiac Grand Prix and the temperature gauge isn’t working. I’ve replaced the battery, water pump, thermostat, and I’ve had the computer reset. What is my problem now?
Answer: Check the gauge circuits, sometimes it's a just a simple faulty ground or the gauge itself that is bad. Go over the tests in the post. That should help to narrow down or find the issue.
Question: What does it mean when my car's gauge registers only when the ignition is off?
Answer: Check the temperature sending unit and, if necessary, its circuit.
© 2019 Dan Ferrell