How Automotive Warning Lights Work

An illuminated service engine light. Unraveling the mystery of the mysterious little lamp is considerably easier -- and usually less painful -- than most people think.
An illuminated service engine light. Unraveling the mystery of the mysterious little lamp is considerably easier -- and usually less painful -- than most people think. See more car safety pictures.
AP Photo/Josh Dickey

Every day we turn the ignition on our cars and go on our way without thinking much about how well our engine is running. But for most cars on the road, each time your car is switched on, it's constantly running checks to ensure that major areas of the engine, electrical systems, emissions and brakes are working correctly. The result of these checks can help you determine whether your car requires maintenance.

Since 1996, all cars sold in the United States are required by the Environmental Protection Agency (EPA) to have an on-board diagnostics system, or OBD. The OBD system works by placing sensors in vital areas of a vehicle, which report back to the diagnostics system whether those areas are working correctly or not. When one or multiple sensors find an area of the vehicle that isn't working within its designed specifications, the sensor triggers a light on the vehicles' dashboard, indicating an area that needs to be addressed.


Think of it like a home's electrical fuse panel. When the fuse panel notices a short in the electrical system on a specific circuit in the home, the panel switches off the electricity to that area that has a problem. In cars, the OBD system doesn't shut anything off, but sends a signal to the driver, via a light on the dashboard, to indicate a potential problem. Automotive maintenance shops can then diagnose problems based on which sensor has been triggered.

Vehicles have been using the OBD system since the 1980s, but since 1996 all light-duty cars and trucks that weigh 8,500 pounds (3,856 kilograms) or less have a second-generation system of on-board diagnostics called OBD-II. The newer system was designed to incorporate a higher standard of emissions testing by the engine, to ensure emission's laws were being met by the vehicle [source: EPA].

But emissions are only one small part of the OBD-II system. Go on to the next page to find out what other types of warning lights can illuminate on the dashboard, and if they're all a signal for future car maintenance.


Types of Automotive Warning Lights

Although vehicles from 1996 and after are required to have the OBD-II system, many vehicles prior to this date were already using warning lights on the dashboard. The lights can vary slightly from vehicle to vehicle, but the most common warning light (and one required by law) is the check engine light. The check engine light or service engine soon light is also commonly known as the Malfunction Indicator Lamp, or MIL.

The MIL is typically a yellow or orange-color light and may display the image of an engine. When the MIL comes on, taking your car to the nearest vehicle maintenance shop may not be immediately necessary; that is, unless the light starts flashing [source: Consumer Reports]. A flashing MIL indicates that there is a serious problem with a part of your engine, or perhaps a potentially serious problem, and immediate auto maintenance is required.


The brake warning light is another common dashboard warning light that will display when an area of the braking system needs attention. The light is typically red, and will usually display the word "brake." If the anti-lock braking system (ABS) warning light comes on, then there's a problem with the ABS and it may shut itself off until the issue is corrected. Normal brake functions can still occur even with the ABS shut-off, but the vehicle should definitely be taken to an automotive maintenance shop as soon as possible.

In addition to the check engine and brake warning lights, there are other important warning lights including the battery warning light, which can indicate that the battery has a low charge, that the alternator is not charging the battery properly, or possibly both. The oil warning light indicates low oil pressure in the engine or that the engine is running low on oil. The airbag warning light denotes that one or more airbags may not function correctly in an accident. The temperature warning light is usually a thermometer symbol and will display when the engine is overheating. Since each vehicle can have different types of warning lights, check the vehicle owner's manual for the most accurate description of each light.

Let's go on to the next page to find out what triggers these lights to come on and the maintenance your car may require.


What Trips an Automotive Warning Light?

In her Automotive Technology class at Wenatchee Valley College in Wenatchee, Wash., Amber Horn uses a scan tool to check over various systems in a 1995 Ford Windstar on April 14, 2004.
In her Automotive Technology class at Wenatchee Valley College in Wenatchee, Wash., Amber Horn uses a scan tool to check over various systems in a 1995 Ford Windstar on April 14, 2004.
AP Photo/Kelly Gillin

Like we mentioned earlier, sensors in the car notify the driver of a potential, or already existing, problem. When a check engine light comes on, there may be a problem with the emissions system. The light can be tripped by something as simple as a loose gas cap, or could mean there's a problem with the spark plug wires, a deteriorated fuel injector O-ring, faulty oxygen sensors or possibly that an area of the engine has water in it where moisture should not be.

If the brake light is on, the parking brake may be set, misadjusted, or something more serious may be occurring that involves a trip to the auto maintenance shop. If the light comes on when you apply the brake or stays on continuously, the hydraulic pressure has been lost in one side of the braking system or brake fluid in the master cylinder is very low. Either way, some type of vehicle maintenance will be required. If the ABS and the brake warning light are on at the same time, the vehicle may not be safe to drive and should be taken to an automotive maintenance shop.


When the battery light is on, it usually means that the alternator is not producing enough electricity to supply the car with electricity and to charge the battery at the same time. This means the car's battery is handling the full load of the ignition system, fuel pump, radio, heater, lights and other electrical functions. The battery light can also be triggered if the belt that spins the alternator is slipping, broken, or has come off of the pulley completely.

A temperature warning light is tripped when the engine is overheating. An overheated engine is always a serious issue and may require car maintenance right away. Sensors in the cylinder head will turn on the warning light if it gets too hot, or coolant sensors may notify the driver if the fluid levels are too low for the system to function properly.

The oil warning light will come on if the sensor determines there's low oil pressure, or if the oil level in the pan gets too low. If there's enough oil in the engine, then low oil pressure is the cause of the warning light, which can come from a worn or broken oil pump, clogged oil in the crankcase's pick-up screen or a plugged oil filter.


Are Automotive Warning Lights Accurate?

With all the sensors in the car trying to determine if there are any problems with the vehicle, how sure can a driver be that the warning lights are accurate? The quick answer is: you can be pretty sure.

Sometimes, a sensor may trigger the warning light to come on, but then it quickly goes off. Typically this occurs when the sensor has told the Engine Control Unit (ECU) of a problem, but doesn't notice the problem as reoccurring. When this happens, it doesn't mean that the system isn't working. It usually means that the system found a problem but that it's been corrected or isn't persisting. In either case, unless your owner's manual recommends you do so, it's not necessary to take your vehicle to an automotive maintenance shop if the warning light goes off and stays off.


In order to check the accuracy of the warning light bulbs themselves, each time the car is switched on the warning lights will appear for several seconds and then go off. This allows the motorist to do a quick visual check to ensure all the warning lights are working properly. If a bulb that usually lights up does not come on, simply replace that bulb.

If a warning light comes on and stays on, then most likely there's an issue with the indicated area which should be addressed by the driver or an auto maintenance professional. A repair shop or even a vehicle maintenance supply store will be able to diagnose the problem accurately using an OBD code reader. The code readers plug in to the vehicle's diagnostic system and tell the reader which sensor or sensors have been tripped. Because the reader can pinpoint what area of the vehicle is sending the code, the warning lights provide an accurate picture of what type of maintenance is required.

For more information about automotive warning lights and other related topics, follow the links on the next page.


Lots More Information

Related HowStuffWorks Articles


  • EPA (Environmental Protection Agency). "On-Board Diagnostics: Frequently Asked Questions." Nov. 18, 2009. (Dec. 5, 2009)
  • Consumer Reports. "What to do if the "check engine" light goes on." March 2009. (Dec. 5, 2009)
  • "Engine Temperature Warning Light." (Dec. 11, 2009)
  • Yahoo Autos. "My Check Engine Light is On - What Does it Mean? (Dec. 5, 2009)
  • Yahoo Autos. "My Alternator Warning Light is On - What Do I Do?" (Dec. 5, 2009)
  • Yahoo Autos. "My Brake Warning Light Is On - What Does It Mean?" (Dec. 5, 2009)
  • Yahoo Autos. "My Oil Pressure Warning Light is On - What Should I Do?" (Dec. 11, 2009)