Why does my temperature gauge stay low? Is my thermostat stuck open?

Get it checked soonUpdated October 24, 20269 sources3 min read

Short answer

If the temperature gauge stays near C, or drops on the highway, after you've driven long enough to warm up, the most common cause is a thermostat stuck open: coolant flows through the radiator all the time, so the engine never reaches its normal temperature. The other usual causes are a faulty coolant temperature sensor or gauge, low coolant, or a cooling fan that won't shut off; it isn't an emergency like overheating, but get it fixed soon, because a cold-running engine burns more fuel, heats the cabin poorly and wears faster.

Safety first. A low gauge is far less urgent than a high one, but don't ignore it. If the needle swings from low to high, or climbs past its normal spot, pull over and follow our overheating steps: a thermostat can also stick closed, and that overheats an engine fast. Never open the radiator cap or coolant reservoir on a hot engine; the coolant is under pressure and can cause severe burns.

On this page

Step by step
What the thermostat does
Why the gauge reads low: the likely causes
Low gauge vs. high gauge
What a cold-running engine costs you
How a shop confirms it
Follow-up questions
Sources

Step by step

1
Drive long enough to warm up
O'Reilly says the needle should start at C and rise gradually to about the middle as the engine warms. Give it a real drive, not a few minutes of idling. Samarins says that after about 10 minutes of driving, the engine should be at roughly 180-220°F (82-105°C).
2
Watch what the needle does on the highway
If it reaches normal in town but falls toward C at highway speed, especially in cold weather, that's a classic stuck-open thermostat: the extra airflow through the radiator cools the engine more than it can heat itself. Samarins lists exactly that symptom for code P0128.
3
Check the heater
Turn the heat to full hot after the drive. Weak, lukewarm heat along with a low gauge points to the engine really running cold (thermostat). Hot heat with a gauge stuck on C points more to the temperature sensor or the gauge itself.
4
Check the coolant level with the engine cold
Low coolant is on O'Reilly's, Edmunds' and Samarins' lists of causes. Look at the reservoir against its MIN and MAX marks once the engine has cooled. If it's low, top it up with the coolant your owner's manual specifies and have the leak found.
5
Look for a cooling fan that never stops
O'Reilly says electric radiator fans generally shouldn't run until the engine is warm. If they run from a cold start, all the time, or even with the engine off, suspect the fan relay or coolant temperature sensor.
6
Scan for codes
A stuck-open thermostat often sets code P0128 and turns on the check engine light. O'Reilly offers free code scans at its stores. With a scan tool that shows live data, the coolant temperature should read close to the outside air temperature on a cold engine and climb steadily as it warms (Samarins).

What the thermostat does

The thermostat is a valve between the engine and the radiator. Jalopnik describes the usual design: a sealed wax element expands as the engine warms and pushes the main valve open to send coolant to the radiator, and a spring closes it again. When the engine is cold the valve stays shut, so the engine warms up quickly; NAPA says that’s the whole point of it being closed at startup.

NAPA says thermostats fail three ways: weak, stuck open, or stuck closed. Contaminated coolant (mixed types, old coolant, debris) and plain wear, like a tired spring, are the usual reasons.

Why the gauge reads low: the likely causes

Cause	What you notice	What fixes it
Thermostat stuck open	Gauge slow to rise or stays low; drops on the highway in cold weather; weak heat; often code P0128	Replace the thermostat
Coolant temperature sensor or wiring	Gauge stuck on C but the heater gets properly hot; may set P0128	Test and replace the sensor or repair the connector
Temperature gauge itself	Gauge dead or erratic while the engine runs normally	Diagnose the gauge or instrument cluster
Low coolant	Low reservoir level; heat may cut out at idle	Top up, then find the leak
Cooling fan that won’t shut off	Fans run from a cold start or with the engine off	Check the fan relay and temperature sensor
Engine computer software	P0128 on a model with a known bulletin	Dealer reprogramming

Causes compiled from O’Reilly, Edmunds and Samarins.

A gauge that swings up and down a lot is its own warning sign: NAPA lists unusual temperature swings as a symptom of a failing thermostat. If it ever swings high, treat it as overheating.

Low gauge vs. high gauge

O’Reilly’s rule of thumb: a needle on C is usually less serious than one on H. Low means the engine runs cold and inefficient. High means it’s overheating, which is what wrecks engines. The Nissan Juke 2025 manual, like most, says the needle should stay in the normal range, which can shift a little with outside temperature and driving, and that if it goes past normal you should stop as soon as it’s safe.

What a cold-running engine costs you

Fuel. O’Reilly, Edmunds and NAPA all list worse fuel economy.
Heat. The heater core runs on hot engine coolant, so a cold engine means weak cabin heat. Firestone says a stuck-open thermostat can stop the heater from blowing hot air, idling or driving.
Wear and emissions. Edmunds says thicker oil lubricates less well and incomplete burning raises emissions. NAPA says over years it can shorten engine life.
The check engine light. P0128 will keep the light on. Clearing the code only turns the light off until the fault sets it again (O’Reilly).

How a shop confirms it

A shop will check the coolant level and look for leaks, read the codes, then watch live data: coolant temperature should climb steadily from near outside temperature to the normal range as the engine warms (Samarins: about 180-220°F after 10 minutes of driving). If it climbs slowly or stalls short of normal while the sensor reads correctly, the thermostat is the usual fix. They’ll also check for automaker service bulletins, since some models have a software update or an updated thermostat part for P0128.

If you replace the thermostat yourself, use the temperature rating your engine was built for. NAPA notes the stamped rating is where it begins to open, not where it’s fully open. Many modern engines use an electronically controlled thermostat (Samarins), so ask the shop which kind yours has before buying parts.

Related: if your heat is also weak, see why the heater blows cold air. If the gauge goes the other way, follow what to do when your car overheats.

Common follow-up questions

What does code P0128 mean?

P0128 is "Coolant Temperature Below Thermostat Regulating Temperature": the engine computer saw the engine take too long to warm up, or never reach its expected temperature. The most common cause is a thermostat stuck open. Edmunds, O'Reilly and Samarins also list a faulty coolant temperature sensor or its wiring, low coolant, a radiator fan that won't turn off, a faulty intake air temperature sensor, and engine computer software that needs an update.

Is it safe to drive with a thermostat stuck open?

For a short while, usually yes; it's much less dangerous than overheating. Edmunds calls short-distance driving generally safe but says not to ignore it for long, and O'Reilly recommends not driving on a P0128 until it's diagnosed. A cold-running engine burns more fuel, puts out more emissions, and its oil stays thicker, which Edmunds says reduces lubrication and increases wear. NAPA notes it can shorten engine life over the years.

Will a stuck-open thermostat hurt my gas mileage?

Yes. O'Reilly, Edmunds and NAPA all list reduced fuel economy as a symptom.

What's the difference between a thermostat stuck open and stuck closed?

Stuck open means coolant always flows through the radiator, so the engine runs cold, warms up slowly and the heat is weak. Stuck closed means coolant can't reach the radiator, so the engine overheats, and NAPA says that can happen quickly and cause major damage. Stuck closed is the emergency: if the gauge climbs, pull over.

Can a bad temperature sensor make the gauge read low?

Yes. O'Reilly says a needle that stays on C can mean a damaged gauge or temperature sensor, not just a thermostat. If the heater blows properly hot but the gauge never moves, the sensor, its wiring or the gauge is the better suspect. A shop can compare the sensor's reading on a scan tool with the engine's actual temperature.

How do you test a thermostat?

NAPA describes a bench test: with the thermostat out of the car, heat a jar of water to at least 20°F above the temperature stamped on the thermostat, then lower the thermostat in; a good one opens, then closes again as the water cools. NAPA also says a thermostat that is open at room temperature is bad. Heat the water by itself, never the thermostat in a microwave, and never open a hot cooling system to look.

Should I replace the thermostat when I get code P0128?

Usually, once the basics check out. Samarins says that when P0128 is set the thermostat is usually replaced, because a thermostat that sticks only now and then is hard to catch in a test. Some models have known fixes first: Samarins lists automaker bulletins for engine computer reprogramming on several GM, Nissan, Mazda and Chrysler models and a thermostat-and-gasket fix on 2013-2014 Subaru Legacy, Forester and Impreza. Ask a dealer or shop to check for bulletins on your car.

Sources

General information only, not a diagnosis. Your owner’s manual comes first. How we write these