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Why Is My Check Engine Light On? Decoding Common Warning Lights

There may be several lights in your car’s instrument cluster, but the one that always seems to stand out is the one you don’t want to see: the check engine light. You can drive for a long time without seeing one light up, but when you do, there are steps you can take.

The thing about the check engine light is that it can go off for a variety of reasons. It’s there to warn you that something isn’t right, but beyond lighting up, it won’t give you much more information than that. Ignore it and you could put your vehicle, or even yourself, in danger.

At the same time, just because it lit up doesn’t mean that the issue is always an urgent threat. It could be something as minimal as a loose gas cap, but whatever it is, it’s best to check it out to make certain it’s nothing serious or pressing.

What do check engine light colours or actions mean?

The check engine light doesn’t always light up in the same colour. It can appear as red, orange, or yellow. In some vehicles, a red light indicates a serious problem that should be checked out at your first opportunity, or may even require you get your car towed. Other vehicles will have a blinking light to indicate the same thing. Solid orange or yellow lights are less serious, but you should still get it checked to find out what caused it.

What causes the check engine light to come on?

A loose cap on your fuel tank is a common reason for the light to turn on, and your first clue is if it comes on right after filling up at a gas station. Always check the cap first, it’s the easiest thing to do and doesn’t require any technical help to figure out. If the cap is somehow defective, replacing it isn’t expensive and is an easy job for any mechanic.

Other reasons for the light’s presence relate to specific components under the hood. Here’s a list of some of the most common check engine warnings:

1. Oxygen sensor (or O2 sensor)

The oxygen sensor monitors and measures how much fuel the car burns. Most vehicles have between two and four oxygen sensors, and if any of those are faulty, it could trigger the engine light. A faulty sensor will reduce gas mileage and increase emissions, but left unchecked, it can damage the catalytic converter, which is an expensive repair costing at least $1,200. Swapping out the oxygen sensor is far less intrusive at an average cost of $260, and may be something you can do yourself.

2. Catalytic converter failure

A catalytic converter plays a critical role in the car’s exhaust because it turns carbon monoxide from the engine into carbon dioxide through the exhaust system. It’s not a complicated part but it’s expensive to fix, so maintaining it is important. Getting regular oil changes helps keep it working properly. There may be other components affecting the converter’s operation, like a faulty oxygen sensor or spark plugs.

3. Spark plug or ignition coil failure

Ignition coils generate the electricity spark plugs use to ignite the engine’s cylinders, so if there is any misfire in that process, that could be enough to turn the engine light on. Note that diesel engines don’t have coils or spark plugs, so this particular issue is for vehicles running on gasoline.

When spark plugs get worn out, you may notice that your car is slow to accelerate. Faulty ignition coils can also cause similar issues, so if you think it’s a spark plug, it may be the ignition coil.

4. Airflow sensor failure

The mass airflow sensor keeps track of how much air enters the engine and also tells the car’s computer how much fuel should go in. If it starts to malfunction, it could lead to rising emissions, or even stalling.

One of the leading causes is the air filter. If not properly installed or never replaced, blockages in the filter disrupt airflow causing the sensor to fail. The mass airflow sensor is a fairly expensive replacement at around $400, and you can avoid it by routinely replacing the filter.

5. Replace the thermostat

The thermostat works like any typical home version, except that it regulates the flow of coolant to the engine. If the coolant has become contaminated with dirt or some other substance, it could turn corrosive, leading to an overheating engine.

Though not exactly the same thing, a coolant leak could also trigger the light once the engine begins to heat up too much. It’s easier to catch a leak because greenish drops or puddles will show up after you’ve parked for a longer period.

6. Replace exhaust gas recirculation (EGR) valve and clean all EGR ports

The exhaust gas recirculation, or EGR valve, maintains a lower level of nitrogen oxide coming out of the car’s engine by redirecting exhaust gas back into the combustion chambers to cool it. Most modern cars have an electrically-controlled valve that opens and closes from the onboard computer. If the flow is off in some way, that could trigger the engine light.

If carbon builds up, it could start sticking, thus clogging up the system. You may notice it by inspecting it visually under the hood, but a clue would be when feeling or hearing a stall or ‘hiccup’ while idling. It is possible to clean out the valve, though it’s not easily accessible in every car. If the buildup is too high, replacing the valve plus a new gasket can cost around $500, depending on parts and labour.

What is an OBD-II scanner?

Taking your car to a mechanic for a minor issue can cost you both time and money. You could do some detective work of your own with the right tools. Every vehicle manufactured since 1996 includes a port for the onboard diagnostic system, which is abbreviated as OBD-II. That port is usually situated under the steering wheel, though some older models may have it on the passenger side.

handheld ODB II Scanner is an onboard diagnostic system, used to diagnose check engine light issues

(Photo Credit: Adobe Stock)

How does an OBD-II scanner work?

When the right scanner is plugged in, the port can give you context on why the check engine light is on through diagnostic or trouble codes. Dealerships and mechanics have computers that do the job, but OBD-II scanners are readily available through retailers, and they work with compatible apps on your smartphone. If you have one in hand, here’s what to do:

Follow the instructions to pair your scanner with your smartphone via Bluetooth.

Make sure the ignition is off and look for the ODB-II port. It’s a standard port that looks the same in every vehicle.

Plug in the scanner and turn on the ignition. Wait for it to finish its initial lighting sequence and follow the steps on the scanner’s app to read the codes. If your scanner doesn’t have a dedicated app, there are third-party ones available in the App Store and Google Play that work with most aftermarket scanners. Some apps are great in not only reading the codes but also highlighting potential issues that could be causing the problem.

Understanding what the check engine codes mean

If you’re not using an app that offers explanations for the codes in plain English, reading the codes requires an understanding of the numbers and letters.

Let’s take the example of P0171-P0175 (oxygen sensor code) and break it down:

  • P stands for Powertrain (B for Body, C for Chassis, U for Network)
  • 0 is for everything generic (unless it’s 1 for a Manufacturer Specific Code)
  • 1 indicates a fuel and air metering issue
  • 71 is the fault code

The third digit is important because it highlights what part of the car has a problem. This is what the numbers stand for:

  • 1 = Fuel and Air Metering and Emissions Control
  • 2 = Fuel Metering
  • 3 = Ignition System and Misfires
  • 4 = Exhaust Monitoring
  • 5 = Vehicle Speed Controls and Idle Control System
  • 6 = Computer Input/Output Circuit
  • 7 = Transmission
  • 8 = Transmission
check engine codes explained- how to interpret codes from an ODB II scanner.

Clearing the code

The better scanning apps will let you clear the code, but consider it a temporary solution. Dealerships and mechanics can do the same thing, though it won’t fix the underlying problem causing it.

Another manual method is to disconnect the car battery, but do it at your own risk. Reconnecting it may reset the light, but also reset all your radio and infotainment settings.

What if the light still comes on?

If the check engine light routinely pops back up even after you’ve tried to fix it, your car may be trying to tell you that it’s on its last legs. And when repairs for an older vehicle are costly, it might be time to upgrade.

Aug 27th, 2019