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.

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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.

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