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Interpreting Dashboard Warning Lights


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Automotive lore calls them idiot lights. For years, critical engine functions were monitored by gauges. Oil pressure, engine-coolant temperature and generator/alternator operation were displayed for easy viewing by the driver.

Why the auto manufacturers initially decided to switch over to warning lights is anybody’s guess, but the motoring public in general was quick to express their disdain over it. Since these lights quickly earned the reputation for being useless – or not conveying warning over a bad situation until damage had already been done – they just as quickly earned their less-than-endearing nickname.

In the mid-1930s, Hudson was the first automobile manufacturer to use warning lights instead of gauges, and it wasn’t long before others followed suit. They quickly became standard, with performance vehicles being the exception with optional gauge packages.

Today, warning lights are still the standard, but it’s a completely different playing field. Oil pressure, coolant temperature and charging-system operation are still the three most important factors, and gauges for these systems have seen a resurgence in popularity, especially in trucks. However, the simplicity of only having those three systems to monitor is long gone.

Today, if it was theoretically possible to have an informative gauge for each and every system on a car, dashboards would look like the cockpit of an airplane. Possible or not, it isn’t realistic, and on top of that, computer technology has made warning-light function accurate and dependable – a far cry from the “idiot” lights of old. In most cases, warning lights today will notify you of a “condition” long before it becomes a problem, and even if it’s one of the three big factors, they’re accurate enough to give you plenty of time to get off the road and shut down to avoid a catastrophic failure.

The hard part is with the number of different warning lights today, it’s hard to know what they all mean, especially with continuously changing technology. And have you seen the size of owner’s manuals? Looking up any one item can send you on a wild goose chase bouncing around through multiple pages to maybe and only maybe find what you’re looking for.

In addition to the growing number of different symbols and warnings, there also are many colors in addition to red. Most manufacturers have incorporated the green-yellow-red idea since we’re used to what that means in relation to traffic signals: green, go; yellow, caution; red, stop. But don’t be surprised to see blue and white thrown in.

Luckily, the symbols for most warning indicators are standardized across all makes and models, making them easily recognizable. But there are still a lot of them. If there’s a light on, there’s a good possibility your customer will ask about it. Here’s a list to help you sort through them.

Check Engine/MIL/Service Engine Soon

This is the main one. The malfunction indicator lamp (MIL) has seen a few different variations over the years, including simple versions with the text “Check Engine” or “Service Engine Soon,” and the now widely recognized engine symbol with a lightning bolt in the middle.

An illuminated MIL means the engine control unit (ECU) recognizes there is a problem with one of the monitored engine systems, which can affect emissions, drivability or dependability. When the light is on, one or more trouble code(s) will be stored by the ECU.

A scan tool is required to access and read these trouble codes, which allow a technician to follow a specified path for diagnosis. An illuminated MIL won’t go out until the problem has been corrected or the trouble code has been erased. However, depending on the problem, if the code is erased without performing any repairs, the code may reset immediately, which will in turn cause the light to immediately come back on. In some cases, the light may take a day or two to come back on.

If the problem is corrected and the code isn’t erased (for example, your customer makes the repair but doesn’t have a scan tool), the code will be erased by the ECU and the light will go out. But, this may take as long as a week, depending on the driving habits of the vehicle owner.

If the MIL is blinking, it means there’s a problem that may damage the catalyst, and the vehicle should be immediately taken in for service. This is generally the result of a cylinder misfire, and a blinking MIL is almost always accompanied by a noticeable symptom of the engine or vehicle shaking.

Another interesting aspect of the MIL is its key-on/engine-off function. When you turn the key on with the engine off, the MIL should illuminate. On some vehicles it may shut off after a few seconds. This is normal. If the light begins to blink, this is an indicator that the readiness monitors aren’t complete, and the vehicle isn’t ready for an E-Check. Of course, readiness monitors are a subject all on their own, but this can be useful to know for anyone in an E-Check area.

Odometer-Triggered Warnings

As emissions regulations were tightening prior to the advanced computer controls we’re used to today, some manufacturers began to use odometer-triggered warning lights for emissions-related systems as early as the mid-1970s. Many of these were oxygen-sensor or EGR (exhaust-gas recirculation) warning lamps, designed to come on at a specified interval for service or replacement of specific parts.

These are rarely seen today, and OBD II, which became mandatory on all cars in the United States in 1996, hastened the end of these types of warnings. If you happen to see one, keep in mind on older vehicles that there’s no scan tool to reset them. There’s a specific procedure that differs per manufacturer, and it’s often a mechanical reset, sometimes on the back of the odometer itself or in a “trick” location.

Battery Light

Here’s an easy one. An illuminated battery means there’s low system voltage for some reason. Is it the battery, the alternator, wiring or a broken belt? Bottom line: When this light comes on, you better get where you’re going quickly – especially at night – or you’re going to run out of power.

Cooling System

The thermometer in water is widely recognized as a cooling-system warning, but it can mean many different things. Red means trouble no matter what, but it can mean that the coolant level is too low, or the engine temperature is too high. These also can flash, indicating there’s another malfunction within the cooling system. On some vehicles, this light also may be green or blue when the vehicle is cold, letting you know to let the engine warm up before any hard acceleration.

Brake-System Lights

The common symbol for brake systems is a circle with an arc on each side. While I’ve never had any documented proof, I picture it as the circle indicating a brake drum or rotor, and the arc on each side indicating the brake linings that act upon them. There are multiple different variations of the symbol.

Red is bad. An exclamation point in the middle means the fluid level is low or the system has lost brake pressure. A ”P” in the middle means the parking brake is engaged.

There also are multiple yellow variations of the light. The letters “ABS” in the middle means there’s a malfunction with the antilock braking system. When the arcs on the side are dashed, this indicates the brake-pad linings are worn very low. This usually is a feature only on higher-end vehicles. If there’s a crossed-out bulb in the middle, it means there’s a brake light out, or a problem with the brake-light circuit.

A green indicator with a foot in the middle means you must step on the brake before you can shift the vehicle out of park or start the car if it’s a push-button start. Most brake warning lights occupy their own spot on the instrument cluster, with the exception of the fluid-level/brake-pressure warnings, which generally utilize the same lamp.

Oil Pressure/Oil Level

The oil can is another one of the three big ones. Red means low oil pressure, and other than when the ignition itself is on, this light should go out when the engine is running. Low oil-level warnings are sometimes indicated by this light in yellow, or sometimes by a separate warning.

Tire Pressure

Easily as well-known as today’s “Check Engine” light is a low-tire warning light, part of the tire-pressure monitoring system (TPMS). It means you have a tire low on air, but if the light flashes initially, it means there’s a problem in the system – usually that one of the sensors isn’t communicating with the system – and it should be diagnosed by a professional technician.

Seatbelt/SRS/Airbag

These are symbols that everyone knows. The seatbelt warning symbol was one of the first to appear in cars in addition to oil, temperature and charging. The airbag symbol means there’s a problem with the supplemental restraint system (SRS). But these days, the SRS is a lot more than just the airbag in the steering wheel. It could be the passenger airbag, side or curtain airbags, seat belt pre-tensioners or more. There’s a lot to these systems and they shouldn’t be taken lightly. Your safety – and potentially your life – may depend on the operation of this system.

Transmission Warnings

The transmission warning light is shaped like a gear and can have an exclamation point inside or a thermometer inside. Red with a thermometer indicates a transmission overheating problem and yellow usually a shifting or gear-ratio problem.

Service Required

Many vehicles utilize lights or warnings to indicate that a regular service is due, from an oil change to a tune-up, or even inspection services. These vary widely between manufacturer, and are often simple text messages, or in some cases just the image of a wrench.

ABS/Traction-Control Systems

Even though we already covered ABS warning lights, it’s common for these to come on in conjunction with traction-control system (TCS) warning lights since the two systems generally share data from wheel-speed sensors. If there’s a problem with one system, there’s likely a problem with the other, or an ABS problem often causes the TCS to shut off, again due to the sensors. Many vehicles have the option of turning off the TCS, in which case the light illuminates by itself with the word “OFF.”

Diesel

Diesel engines have a few additional warning indicators that differ from gasoline engines. The glow-plug light, which looks like a couple loops of wire, illuminates either when the system is operating, or if there’s a malfunction. In most cases, if there’s a malfunction, there also will be a trouble code and the standard MIL will be illuminated.

Newer diesels have warning lights to indicate when the diesel-exhaust particulate filter (DPF) needs service, and when the diesel-exhaust fluid (DEF) is low. These lights vary widely in appearance, and some of them look downright silly like a third-grade art project. But when you look closely, you’ll see they’re trying to depict particles, fluid and the movement of exhaust.

Unless the car is new to them, diesel owners will be well-aware of the DEF light since the vehicle won’t run when it’s out of fluid.

Washer Fluid Low

This is an easy one: The view you have with the wipers on, fluid and a fountain.

Exterior-Light Warnings

There are many different exterior-light warnings. The most common is the age-old blue “high-beam” indication, but some vehicles feature this warning in green to let you know the low-beam headlights are on, or they just have the letters “DRL.” A bulb-out warning is the image of a bulb; the bulb inside the universal sign for brakes indicates a brake lightbulb out.

Many new cars have adaptive headlights, meaning they self-adjust to compensate for vehicle loads and steering. A red warning with arrows indicates a malfunction with this system.

Door/Trunk/Hood Ajar

This one is always red. Most today depict the exact location of the offender. Some are just text.

Low Fuel

We all know what this means. Sometimes it’s just a round light that comes on as the gauge nears “E.”

Drivetrain Warnings

There are probably more variations of these symbols than any other. “O/D” stands for overdrive and indicates the overdrive is turned off or disabled for some reason. If there’s a malfunction in a system that causes the overdrive to be disabled, there will most likely be an illuminated MIL to accompany it.

All-wheel-drive (AWD) and four-wheel-drive (4WD) systems that can be controlled by the driver often have text indications of what’s engaged and what gear range, or the universal symbol indicating front and rear differentials and a center transfer case is commonly used as well.

Full-time AWD systems just utilize the lamp when a problem prevents proper system operation, and depending on the type of system and the inputs it receives, there may be other warning lights on at the same time such as the MIL or
ABS indicators.

ADAS

Advanced driver-assistance systems (ADAS) came along with their share of indicators to warn us of lane departure, distance warnings, forward collision avoidance and blind-spot monitoring.

And More!

It can seem like a never-ending list. “EPC,” common on some European cars, stands for electronic power control and means there’s a problem with the throttle system. A key indicates a problem with the vehicle anti-theft system, and depending on whether it’s steady or flashing, it can clue you in to the problem.

A steering wheel with an exclamation point means there’s a problem with the power steering. A snowflake means the temperature is close to or below freezing and there could be ice on the road. The cruise-control symbol is a small speedometer with an arrow pointing to a specific spot, sometimes white or yellow to indicate the cruise control is on and green to indicate it’s set.

The bottom line? You still may need to go on that wild goose chase in the owner’s manual to determine the exact meaning of any given indicator. And what about electric vehicles, you might ask? Yes, they have them. That’s for another day.

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