Temperature Gauge Goes Up and Down While Driving (The Causes)

Just like all breathing and living creatures, cars also have optimal working temperature under which they function normally. With that in mind, a car’s working temperature must remain constant, and any big fluctuations could harm your engine in various ways.

As soon as you start the engine, friction and detonations inside the cylinders create enormous amounts of heat. Each part of the engine is designed to tolerate a certain amount of heat, and prolonged exposure to extreme heat can cause serious damage or even destroy the engine.

That’s why cooling system is among the most vital ones for your car’s good health. The system consists of a water pump, thermostat, radiator, radiator fan and plumbing which sends water and coolant mixture throughout the engine.

This system is designed to maintain the ideal working temperature of 190 Fahrenheit no matter the outside temperature. However, as all car parts wear, some malfunctions can cause your engine’s temperature to either go up or down, or even fluctuate during driving.

You need to know that a certain range of fluctuation is normal, but if the temperature fluctuates wildly, you’re looking at a problem with your cooling system.

Depending on the symptoms, there are different parts of your car’s cooling system that could go wrong. Luckily for you, most of them are easy to notice, diagnose and sort out. So, I’ll address these issues and help you pinpoint the breakdowns on time and save your engine from overheating or running cold.

Why does a cars temperature gauge go up and down while driving.

Reasons Why Your Car’s Temperature Gauge Goes Up and Down

There are several reasons to why your car can’t keep the ideal working temperature. These are the most common issues you can encounter.

Faulty Temperature Gauge

This is the simplest issue to fix, and the least dangerous one for your car’s health too. In this case, the broken temperature gauge will show temperature fluctuations, while the actual temperature of your engine remains steady.

You can diagnose this issue by yourself via the ODBII reader. By connecting your smartphone via the ODBII adapter and via the designated app, you can easily check the temperature regardless of what the gauge shows. In case you have a faulty gauge, you’re lucky because the engine hasn’t suffered any potential damage due to temperature fluctuation and neither have any parts of the cooling system.

Coolant Issues

If the temperature gauge fluctuates alongside the ODBII readings, be prepared for a set of potential issues. One of them is a clog in the radiator, but there’s also air in the coolant plumbing system.

Coolant leaking somewhere in the system is also a common issue and can be a result of a faulty thermostat (I’ll get on it in details below), damage to the plumbing or damaged radiator.

If the water pump is malfunctioning, your car doesn’t get a steady supply of water and coolant mixture which could cause the temperature to fluctuate as well.

In case any of this happens, it would be the best to contact your mechanic and describe the issue in detail.

Faulty Fan

A radiator fan is another important part designed to keep your car’s temperature on a steady level by cooling the radiator. Erratic temperature fluctuations could be caused by failed fan resistor or relay.

A relay controls the power of the cooling fan, while the resistor controls when the fan turns on or off. In the first case, the fan can turn on and off randomly, stay on all the time or fail to turn on at all. In the other case, the fan can randomly turn on and off.

Both faults can cause drops and spikes in temperature and both faults are easy to repair. A failing cooling fan is just one of the options though, so let’s continue and see another common cause of erratic temperature jumps.

Faulty Thermostat

A thermostat is designed to keep the constant temperature by regulating proper flow water and coolant mixture depending on the engine’s temperature. As such, it is an important part of the cooling system and its failure or faulty operation might cause a set of problems.

If the thermostat is constantly open, it will send excess amount of coolant liquid into the engine. As a result, your engine will fail to achieve its optimal working temperature. That will wear its parts faster, cause more emissions and reduce its overall performance.

However, if the thermostat fails to open, your engine will heat up quickly, but without coolant liquid, it could overheat more quickly. That’s why you should always keep an eye on your temperature gauge and avoid high repair bills.

Some thermostat faults might cause the temperature to climb and drop in mere moments. Usually, this symptom will occur alongside one of the above, pointing out to a failing thermostat. Another good sign that your thermostat is due for replacement is leakage in the cooling system.

If the thermostat fails to open, increased pressure will send the liquid outside of the system. Most commonly, the leak will occur in the thermostat housing, but the plumbing is also in danger of leaking.

Luckily, thermostat replacement is a fairly simple procedure which won’t set you back a lot. Depending on the make and model, you’re looking at a bill that shouldn’t cross the $200 mark, which is significantly cheaper than what you’d pay for reviving an overheated engine.


Fortunately, each issue that could cause erratic behavior of your car’s temperature gauge is inexpensive to sort out and won’t take long to fix either. If you have some mechanical background, you could fix some of the issues yourself, but my advice is to always leave your car in hands of a trusted mechanic.

Also, it’s important to point out that noticing abnormal behavior of the temperature gauge requires acting fast, because many of these issues can cause serious damage that could lead to costly repair bills. Whatever is the reason for sudden drops and spikes in temperature, it’s good to know that you can identify each potential malfunction yourself and save some money whilst saving your mechanic’s time too.


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