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Hazard lights or warning lights are a car feature included in the indicator switch assembly. 

This feature makes both indicators flash together compared to the turning signal only using one. 

Hazard lights can blink randomly when the car is off, though. Let’s explore why below.

Reasons When The Car Is Off

There are several reasons why hazard lights turn on with the car switched off. Some reasons are designed into the car and are useful. Such as indicating the alarm is armed.

Other reasons are faults within the circuits, corrosion, or a faulty hazard switch.

Before we look at possible faults, a quick caveat about normal hazard light operation. I just had to include this for newer drivers.

Normal – Indication

Many car manufacturers add a feature, so whenever you unlock or lock your vehicle, the hazard lights flash, sometimes with the parking lights, to indicate that the arm the alarm command from the car remote is received. 

Another feature put in by the manufacturer includes the flashing of hazard lights whenever a  door opens. Quite a new feature, this one!

Interesting article on what causes car alarms to go off randomly! Car Alarm Going Off Again? 8 Causes and Fixes

It warns the car behind you that something is happening with the vehicle in front. This alerts the driver behind to decide whether to slow down, wait, come to a stop or drive around—a great addition.

Below we’ll help you figure out what might be causing your hazard lights to go off with the car sitting. 

Fault In The Circuit

A set of hard wiring inputs combined with a flasher and sometimes sensors control the hazard lights.

If your hazard lights stay on, blink without command, or blink with an irregular rhythm, it is probably because of a grounding problem in the switch circuit. There is a relay in the switch assembly that has a resistor inside of it. 

This resistor ensures a rhythmic flash or blinking. Grounding faults within the relay cause irregularities in the blinking, blinking without command, or leaving the hazard lights switched on.

It could be a bad turn signal relay or a bad hazard switch and relay grounding.


To check whether it is the grounding or the relay, first unplug the switch to check if the lights go off.

Another way is to check the rhythm of the blinking. Due to varying resistance, faulty relays cause the blinking to become very fast or slow. 

Check the switch assembly and the signal relay if inconsistent blinking occurs on the hazard lights. 

You can also use a voltmeter to check if there is any voltage drop with the car on idle.

The grounding may cause a voltage drop when the hazard lights blink on their own.


After diagnosing the fault, if it’s the relay, then change it or have it changed because it may cause battery drainage overnight.

The relay can usually be found in the dash behind the radio. Sometimes, the radio has to be pulled out, or it can be reached from under the dash. 

In some cars, the turn relay is found in the fuse box, in the cab marked as turn, or under the hood.

Check to see which fuse is burnt, just in case. A car electrician can best check the grounding in the hazard switch circuit.


Minor corrosion won’t be a problem, but corrosion build-up over time can cause issues – light hazard lights coming on and off, staying on, and random blinks.

Such corrosion is common in cool, damp climates or places that receive a lot of rain and humidity.


This would require technical knowledge of the car you drive because a physical inspection needs to be carried out. 

You can check under the hood for the connection points of the hazard lights. Also, check the fuse box in the cab for the hazard switch and the tail lamps for corrosion on the hazard lights.


If you checked the connections and found corrosion, prepare for some cleaning!

First, disconnect the battery, and start rubbing down the metal contact points with mild sandpaper. There are dedicated products for this situation, which might speed the process up.

Faulty Hazard Switch

There is a good chance that the hazard switch is faulty. This is the button you press to turn on the hazard lights. The electrical switch underneath the plastic casing might be faulty. 

There are situations where the button sometimes gets stuck and does not come out after you press it to turn off the hazard lights.

There is a good chance that the toggle behind the button that controls the power supply and the flow of electricity might be faulty and sticking.

There is a gateway that closes when you press the hazard lights button. This enables the flow of power, and the power is cut off when the gateway is open.


You can press the button on and off many times to see if the hazard switch is working like it’s supposed to.

You need to check if the hazard lights turn on and off respective to the press of the button and if the blinking is in rhythm and consistent.


If you have figured out the button is faulty or just getting “stuck,” then take a pin or a skinny blade and insert in the groove or seam of the button and shake it or tug it gently to pull back or release the button completely.

You can also apply some WD-40 to eliminate any debris, moisture, or dryness that might be causing the button to get stuck. 

If the toggle is faulty, then the WD-40 might help, but if it still causes a problem, it will have to be replaced and might cost you anywhere between $70-$120 (parts and labor included), depending on the car and model.

Warning Lights

Modern cars have a new feature that deploys hazard lights as warning lights. Every time ABS (Anti-lock Braking System) is activated, an actuator in the braking system alternates the braking pressure on individual brakes to ensure that no wheel locks up when braking. 

This uses the ABS actuator to turn on the hazard lights as warning lights every time the ABS kicks in. This is to inform the car behind that there is an obstruction in front and that the driver behind should either slow down, halt, or try to avoid a collision from behind. 

An active speed sensor on each brake conveys speed in real-time to the ECU (Electronic Control Unit) to determine how much braking pressure is necessary to slow down or halt the car completely. 

Sometimes the active speed sensor might become faulty for various reasons and may cause the ABS to kick in unnecessarily.

This results in hazard lights coming on every time you apply your brakes hard. 

Sometimes the active speed sensor is so faulty that it causes the ABS to fail or stay activated unnecessarily, giving the appearance to motorists behind you that you permanently have your hazards on.


A faulty active speed sensor causes the ABS light to turn on in your dash, and if your car has ESC (Electronic Stability Control), then the ESC light also turns on. Sometimes both will turn on at the same time.

You will notice hazard lights coming on every time you brake, even when you brake lightly.

An OBD2 diagnostic tool can also run a diagnosis and tell you if the ABS actuator or the active speed sensor has become faulty.


If it’s a faulty active speed sensor, it could be for two reasons; it could be defective due to mechanical reasons, or it could be faulty due to electrical reasons. Both will send bad messages to the ECU.

Mechanically, a toothed ring on the wheel hub induces a magnetic flux in the active speed sensor. It could be a missing tooth, a crack in the ring, metal contamination on the sensor and the ring, or damage on either one. 

If it’s contamination, then a service or clean-up can fix it. There are dedicated products for cleaning sensors and contamination.

If it’s damaged on the ring, it must be replaced, and it can set you back $60 – $300. The ring costs around $10 – $50, but since it is laborious, the labor costs between $50 – $200, depending on the car and model.

For electronic reasons such as shortened sensor coils, weak magnets, faulty wires, and a blown fuse, replacements must be made.

Anything concerning the sensor will require replacing the sensor – $10 – $100. Faulty wires will require rewiring – $20 – $80, and blown fuse – $2 – $15.

Prices will vary depending on the car and model.

In Conclusion

Sometimes, you will need to check your car’s security system, especially if it’s aftermarket (usually won’t cost anything since they messed up). 

Otherwise, you will need to check the parts individually to ensure proper use of the hazard lights. These are used to ensure the safety of your car and the cars around you, so always get professional help when changing anything or installing any aftermarket components. 

Always make repairs if you have the required knowledge, or seeking professional help will be beneficial, albeit it might cost extra. 

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