Among the number of lighted alarms, pings, and dashboard buzzes that accompany today’s cars, the most frightening of all has to be the check engine light. Part of this fear is due to the nature of the alarm itself - since aside from illuminating persistently, the thing doesn’t really tell the driver much about what is going on.
While typically triggered by a problem related to the emissions systems, the check engine light can mean many things – from a loose gas cap to a failed sensor to more serious engine trouble. So, let’s take a closer look at the alarm itself to better understand how it works, as well as some of the top reasons it may be coming on.
As Sensors Go: How Does Check Engine Light Apply?
Today’s late model vehicles are typically outfitted with a number of digital sensors that monitor different mechanical operations. Some of these sensors stand alone, like the dash light for the Tire Pressure System (TPS) which illuminates whenever the car’s tire pressures are low. Other dash lights – like the check engine light – involve clusters of different sensors in order to get the job done.
Now, all of these sensors communicate through a digital component known as an Electronic Control Unit (ECU). The ECU serves as a central ‘brain’ that manages and stores operational and performance information regarding the car’s real-time condition. Consequently, if the check engine light illuminates (or blinks, depending on how critical the problem is), this means that somewhere within the sensor cluster a digital condition code has been triggered and sent to the ECU.
Since the light itself does not detail where the specific problem is, an OBD-II reader is used to pull the code and help diagnose what’s going on.
However, even without a scan tool on hand, you can still narrow down the list of possibilities by doing a little investigating. So, to gain a sense of what might be causing trouble, lets meet some of the individual sensors applying to the check engine light.
Sensors That Can Set Off Check Engine Light
Engine Coolant Temperature (ECT)
As one might expect, this sensor measures heat relating to the engine’s coolant. If the temperature goes beyond the ECUs expected value, the sensor will trigger a dashboard light. (It usually looks like a thermometer, or the word “TEMP.”) However, if the coolant temperature sensor is failing or has failed, it may also trigger the check engine light. The ECT sensor is usually located on or near the thermostat or its housing.
Mass Airflow (MAF)
Today’s vehicles need to measure the amount of air entering the engine to ensure optimal performance, but more importantly to gauge proper air-fuel ratio relating to emissions quality. If the MAF Sensor experiences an irregularity, it will trigger an alarm. This component is usually located near the ECU at the top of the engine intake.
Intake Air Temperature (IAT)
Air temperatures at the engine intake require similar monitoring. If the IAT sensor sees higher-than-normal temperatures or if it fails, an alarm will be triggered. As one might expect, this sensor is located on or near the engine intake itself.
Oxygen (O2)
Also helping to regulate the engine’s air-fuel ratio, O2 sensors monitor the amount of unburned oxygen at different points in the exhaust system. Most modern cars have at least two sensors on the exhaust manifold – one in front of the catalytic converter and one after it – but some vehicles may have three or four sensors. If any of them detect an irregularity, an alarm will be triggered and the check engine light illuminated.
Fuel Tank Pressure (FTP)
Part of the evaporative emissions system (or “EVAP” as it’s commonly called), the FTP sensor detects evaporative leaks in the fuel system. If the sensor detects a leak, like from a loose or faulty gas cap, or if the sensor itself fails, the check engine light will come on. This component is located on top of or inside the fuel tank.
As you may have noticed, when it comes to the check engine light, the main areas of focus are temperature, oxygen and fuel. This information should offer you a better understand of where to look next when trying to investigate exactly what is wrong, and how to go about fixing the problem.
Same Light, Different Alerts
Another important bit of information when investigating a check engine light is the behavior of the light itself. Essentially, there are two trouble conditions you need to be aware of:
If the light appears as a steady alert, ‘something’ in the cluster of sensors and/or individual components leading to a sensor alarm has gone south. This means you should be concerned, but at the moment are not facing a critical failure. Basically, you have a bit of time to troubleshoot the problem before the vehicle stops running.
However, if the light appears as a blinking alert, ‘something’ in the cluster of sensors and/or individual components leading to the sensor alarm has failed entirely and is threatening the stability of the vehicle’s operation. As a result, you should stop immediately and take remedial action.
This can take the form of driving to a shop for a full diagnostic checkup, or if you have your own error code reader, immediately stop to plug the reader in and read back the relevant code. Once you know what’s happening, consult a parts-supply provider for a part replacement, or get the vehicle to a professional service provider.
Top Reasons the Check Engine Light May Be On
1.) Loose or Faulty Gas Cap
Due to emissions regulations, most of today’s vehicles are fitted with sealed gas caps to trap fuel vapor before it is injected into the atmosphere. If the cap comes loose, or if the rubber O-ring that seals it with the gas tank neck fails, the FTP sensor can go off and trigger the check engine light.
If this is the case, you’re facing a quick fix of simply tightening the gas cap or perhaps, having to purchase a new one.
2.) Failed MAF Sensor
Being in the engine bay, the MAF sensor gets clogged with contaminants over time and is subject to a lot of vibration. This can cause the sensor to fail, throwing a check engine light and leading to performance issues like poor fuel mileage, stalling and misfiring.
If the MAF sensor is malfunctioning due to excessive dust and oil particles, you may be able to restore its performance by giving it a good cleaning. You must use a dedicated MAF Sensor Cleaner. However, if the problem is due to mechanical fault, electrical error or actual damage to the sensor, replacement is the only option.
3.) Failed O2 Sensor
A malfunctioning or failed O2 sensor is one of the most common reasons for an illuminated check engine light. Since this sensor lives in a pretty harsh environment (your vehicle’s exhaust system), it becomes coated in dirt, grime and other combustion debris that can cause it to wear out. Bad gas or using the incorrect fuel for your vehicle can also cause an O2 sensor to fail.
While your vehicle will continue to function with a bad O2 sensor, it will burn more fuel than usual and could eventually cause damage to other components – like your catalytic converter – so, don’t put off replacement.
4.) Catalytic Converter Issue
A very important part of your car's emissions control system, the catalytic converter traps harmful gasses emitted by the engine and converts them into less harmful substances, thereby creating a cleaner gas plume. If the catalytic converter becomes clogged or begins to overheat, you may notice sluggish engine performance or even a rotten-egg sulfur smell. You may also notice an illuminated check engine light, triggered by an O2 sensor.
A bad catalytic converter is not something to ignore, as it often signals a larger problem and will eventually lead to engine shutdown. This is an expensive part to replace, but keeping up with regular vehicle maintenance (like oil changes) will help prevent its failure.
5.) Bad spark plug
Faulty spark plugs, spark plug wires or ignition coils can cause a constellation of issues including rough idling, poor performance, engine misfire and – you guessed it – a bright, annoying check engine light.
Thankfully, this replacement is both easy and affordable – so don’t put it off, because a delay can lead to more serious engine trouble.
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