Coolant Light Turns On But Tank Is Full: Possible Causes

A lit up coolant indicator on a speedometer.

You may take excellent care of your car, keeping the radiator clean and the coolant tank filled. Suddenly your coolant light turns on but the tank is full, what are the possible causes? Some people will ride around with the light on and just ignore it, keeping constant vigilance on the level of the overflow tank, but this is never a good idea.


Your coolant light will come on when the sensor senses an incorrect change in the temperature in your system. Typically the sensor, which monitors the coolant temperature, knows how to adjust to varying temperatures depending up the status of your car; running, warming up, or cold. The sensor is very important because your car’s systems depend upon it running at defined temperatures. The coolant sensor is also referred to as the master sensor.

Sensor Problems

Your sensor may fail or it may simply give a bad reading. A bad coolant sensor may cause a decrease in fuel efficiency or it may also cause the vehicle to fail your emissions inspection.

It is likely that the problem may be a loose, corroded, or faulty connection and not a problem with the sensor itself. Have all of your connections checked to see if this may be the source of your problem.

Visual diagnoses can sometimes detect a problem with the sensor. You will be able to see if it is cracked or badly corroded. In this case, removal and replacement is necessary. More often than not, though, you will need to take voltage and resistance readings from the sensor to see if it is functioning properly.

If your car is older, the sensor has deteriorated with time and may be failing but not entirely failed. It is not a bad idea to keep an eye on your sensor’s readings. Replacing a sensor that is starting to show extreme signs of age and wear can prevent future problems.

Your Thermostat

You may have some sensor problems that stem from the thermostat rather than the sensor. Your thermostat may be stuck open and causing improper readings. If you recently had your thermostat replaced, check to see that it was replaced with the correct temperature range.

Cracked Block or Gasket

Your coolant light might be signs of a more major engine problem. When you get a crack in the block or the intake gasket you will leak coolant and oil, usually slowly at first. You may not notice the difference at first, thinking that your tank is full

A repair to a cracked engine block or head gasket is an expensive repair. While you drive, take note if you smell coolant. That may be an indication of a leak that you haven’t seen yet. Also watch below your car for any telltale drips.

Coolant Level Module

Your problem also be a faulty coolant level module. Check the wiring to be sure nothing is loose or corroded.

These are some of the possible causes of a coolant light going on when the tank is full. You'll want to take your vehicle to a qualified repair location to diagnose and repair your problem if you can not easily diagnose it yourself.

Coolant Light Turns On When Tank Is Full FAQ

Why is my coolant level low even after filling?

If you notice that your coolant light remains on even after you fill up the tank, there are several reasons why this could be happening. For one, the sensor itself might be bad, or the connection to the sensor may be corroded, damaged, or loose.

Low coolant levels can also indicate that there is a leak in a hose, a water pump, or some other part of the vehicle where the coolant flows.

Check for leaks and check the sensor and the connections to the sensor and you may find the problem that is keeping the coolant light turned on.

What are the symptoms of a bad coolant temperature sensor?

If the coolant light is on, you might not have a leak. This could be an indication that the temperature sensor itself is faulty.

There are several symptoms you may notice if the temperature sensor has failed. You may notice that your car's gas mileage has become worse or that the engine seems to be running poorly or sluggishly.

The engine will also be more prone to overheating if you have a bad temperature sensor. The engine might also stall or smoke, and the check engine light might come on.

Can I drive with coolant light on?

If you notice that your engine coolant light is on, the best course of action is to stop driving the vehicle as soon as possible and turn off the engine.

Unless there is some electrical problem at work, this light indicates that the engine is getting hot. Driving on a hot engine can cause a great deal of damage to your vehicle and create new problems that can be extremely difficult and expensive to fix, such as a cracked engine block.

How long do I have when the coolant light comes on?

When the coolant light comes on, do not try to continue driving. It is strongly advised that you pull over and turn off your engine when this light comes on.

Wait at least 30 minutes for the engine to cool before you attempt to drive again. At this point, you should only be driving to the nearest service station or garage where you or a professional can work on the vehicle and determine why the light is coming on.

Is antifreeze the same as coolant?

There are many different fluids inside any engine. These fluids serve to lubricate engine parts so they can move more smoothly or cool off engine parts that get very hot.

Both antifreeze and coolant are used in engines and both perform a similar function. However, these are not the same chemical compounds.

In other words, do not use them interchangeably. Do not use antifreeze instead of coolant, or vice versa, as these are two separate fluids.