Decoding the Mysterious Check Engine Light

‘Tis the season for holiday lights so merry and bright – unless it’s your check engine light! Some are solid and some may flash, but this is no time to ignore the lights on your dash! When your dashboard twinkles, it may be tempting to put off finding an auto mechanic for diagnosis, but it’s important to the life of your vehicle to understand what this light means and whether it’s time to have your vehicle serviced. A little knowledge about the mysterious check engine light can mean the difference between tightening your gas cap – no biggie – to getting to a garage ASAP before your engine seizes – yikes! 

How Check Engine lights work

All cars today come equipped with a computer and sensors that continually monitor your vehicle. These very sophisticated computer systems manage the engine and all of your car’s components. Multiple sensors around the engine and all its connected systems provide information that is necessary to keep your vehicle operating at peak performance.

Check engine lights (as well as other dashboard warning lights) communicate issues from your vehicle to you and your automotive service technician. A Check Engine signal, otherwise known as a Malfunction Indicator Lamp (MIL) shares the issue via a code and that code gives the technician a starting point when diagnosing the fault.

What do sensors monitor?

Sensor data is digested continually, and is used to control things like the amount of fuel injected into the cylinders, the timing of when the spark plugs ignite it, and how efficiently the fuel is consumed. When the data from one of these sensors gets outside a narrowly-defined range of acceptable values, several things happen inside the computer’s software. In order to let you continue driving the vehicle, the computer may substitute a default value, one that’s close enough to get you home from the grocery store and then to the auto repair shop. Then it sets what’s referred to as a trouble code in its internal memory. There are hundreds of these codes and they are quite specific. These codes trigger indicator lights and provide important information to your auto mechanic. Sensors monitor many things, including:

  • Accelerator Pedal Position Sensor
  • Air Flow Meter/Sensor
  • Air Temperature Sensor
  • Barometric Pressure Sensor
  • Camshaft Position Sensor
  • Coolant Temperature Sensor/Switch
  • Crankshaft Position Sensor
  • Evaporative Vapor Pressure Sensor
  • Fuel Pressure Sensor
  • Fuel Tank Pressure Sensor
  • Ignition Switch
  • Knock Sensor
  • Manifold Pressure/Vacuum Sensor
  • Oxygen Sensor
  • Throttle Position Sensor
  • Transmission Position Sensor/Switch

Less mysterious indicators

You can purchase a car code reader such as the Harman Spark to decode check engine symbols yourself. Flashing lights usually mean that the issue is more urgent than solid lights.

Some symbols are easier to decipher than others, such as the oil and battery indicators. If the oil light comes on, there may not be enough oil moving through they system. It could also mean that the oil pump isn’t pumping the oil through properly. You can check the oil as a starting point to see if it’s simply too low. If you have plenty of oil, your next step should be making an appointment with a professional auto mechanic.

The generic “Check Engine” light leaves a lot to the imagination. However, you can check on a few things before heading to a service center. The most common reason for an illuminated Check Engine light is a loose gas cap. You can fix that one yourself! A loose or faulty gas cap can render a code labeled as either P0457 or P0455. Once tightened, it may take a few key cycles without the error before the check engine light will turn itself off.

An illuminated battery light means that the battery isn’t being charged sufficiently. The alternator and voltage regulator work to keep the battery charged and run all of your accessories. If the light is on, get to a service center before it dies.

If you discover that the problem is complicated or beyond your skill level, schedule an appointment to have your car checked by an automotive technician.

When in doubt, find a reputable auto repair shop

There’s no need to go it alone. The technician will plug a scan tool into the diagnostic port under your dashboard, read the trouble code(s) and examine freeze frame data (a snap shot of what was going on at the exact time the code was set). This will give the technician a starting point for their diagnosis. Once the issue is resolved, they’ll use the scan tool to erase the codes and turn off the light.

There is real risk in relying on car engine codes for diagnosis. Just because your computer returns an engine trouble code for a poorly-operating MAF (Mass Air Flow), doesn’t necessarily mean there’s a problem with the sensor. The issue could simply be a loose hose or leaky ducting between the MAF sensor and the throttle. That’s important because the MAF sensor could cost hundreds of dollars. Tightening a leak takes only a minute and shouldn’t cost more than the shop’s minimum charge for diagnostics. There are plenty of other sensor-related trouble codes that could well be caused by something other than a bad sensor. It’s best not to guess.

What if my car is running OK with the Check Engine light on?

Do not ignore check engine lights. The “out of sight, out of mind” treatment is the worst way to treat your vehicle if you want it to last. Even though your car may seem fine, it can enter a different regiment when there is a problem that triggers a check engine symbol. It no longer self-tunes for the best economy, mileage, power, and emission control like it’s supposed to. Although it may be unnoticeable, you’re getting fewer miles per gallon as well as reduced ability to accelerate up a freeway ramp or complete a passing maneuver. The moral of this story is: you owe it to your car and yourself to get the check engine symbol checked! Maybe that can be your New Year’s resolution.

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