What Is a Check Engine Light?
Every car and light truck sold in the U.S. has an onboard diagnostic (OBD) system. When it detects a malfunction – usually one that could increase emissions – it activates the Check Engine Light on the dashboard. It stores an electronic code that indicates where the problem occurred.
What Does the Check Engine Light Look Like?
The Check Engine light is a yellow, orange, or amber dashboard light that illuminates for a few seconds every time you start the engine to show that it’s working. It can be in the shape of an engine or, say, “Check Engine,” “Service Engine Soon,” or something similar. It should come on briefly with all of the other warning lights when the ignition is turned to “On.”
Is the Check Engine Light Flashing?
If the Check Engine light is flashing, blinking, or is red instead of amber, there’s a serious problem, such as a misfire. Stop driving as soon as possible and have the cause diagnosed by a mechanic. If you’re halfway through a cross-country trip when the light starts flashing, don’t press on and hope you make it to the finish line. Continuing to drive could damage an expensive part, such as the catalytic converter.
However, if the light stays on in a steady illumination, don’t panic. It may be on because of a loose gas cap because it’s time for an oil change or some other routine maintenance. It’s probably safe to continue driving unless the engine hesitates, feels sluggish, or shows other signs of distress. However, have a mechanic find out what triggered the warning light immediately. Don’t ignore it; it’s unlikely to turn off on its own, and the problem that caused it won’t go away either.
Reasons Your Check Engine Light Might Be On
There are literally hundreds of reasons that could trigger a Check Engine light, but here are some of the more common ones:
- Loose or worn gas cap: Failure to fully tighten the cap after fueling or a worn cap that doesn’t tightly seal allows evaporative emissions.
- Bad oxygen sensors: Most vehicles have two that measure the amount of oxygen in the exhaust system to tell the engine management computer how to adjust the air-fuel mixture. If one of the sensors conks out, the computer will receive incorrect or insufficient information.
- Mass airflow sensor: The engine computer also decides how much fuel to inject into the engine based on how much air is coming in. If the airflow sensor in the intake system (located ahead of the air filter) fails or gets clogged, the computer won’t be able to set the correct air-fuel mixture.
- Catalytic converters: Catalytic converters clean up exhaust gases and are expensive to replace, from about $1,000 to $2,000. When a “cat” gets clogged or damaged from overheating or road debris, it needs to be replaced.
- EGR system: This is the Exhaust Gas Recirculation system, where some exhaust gas is taken back through the engine to reduce tailpipe emissions. If the EGR valve fails or the system gets clogged with gunk, the Check Engine light can be triggered.
- Clogged or dirty fuel injectors: Sediment from the fuel tank, crummy fuel, or carbon buildup can clog fuel injectors, and engines with direct fuel injection are more susceptible.
- Faulty spark plug: A bad spark plug can cause misfiring, damaging internal engine parts, and can cause the Check Engine light to flash. Spark plugs typically last 100,000 miles or more, which is more likely on high-mileage vehicles.
- Worn or loose vacuum hoses: Vehicles have dozens of small vacuum hoses that relieve pressure in the engine and supporting systems. If they become loose or develop even tiny pinholes, the leaks can cause an engine to run roughly, be hard to start, and even backfire – and the trouble spots can be hard to find.
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How to Fix a Check Engine Light
The Check Engine light can be triggered by something as simple as not fully tightening the gas cap. Check the gas cap first and cinch it to ensure it is tight. If it is loose, the Check Engine light should go off after the engine has been started several times.
If the gas cap isn’t the culprit, then an OBD code reader (or scanner) needs to read the system that plugs into a connector under the dashboard. Any auto repair shop can do this – for a fee. Many auto parts stores will scan your OBD system for free, or you can buy a code reader for around $50 and do it yourself. Inexpensive code readers, though, provide only limited information.
Should You Reset Your Check Engine Light?
Though you can turn off a Check Engine light with an OBD scanner, it will revert if the problem hasn’t been fixed.
What Does an OBD Scanner Tell You?
When you plug a scanner into the OBD system, it will search for diagnostic trouble codes (DTCs). These are five-digit alphanumeric codes that identify where the issues are occurring. For example, the P0440 DTC points to the evaporative emissions system and PO401 to the exhaust gas recirculation system.
The “P” indicates that the problem is related to the powertrain, the engine, and the transmission. If the first number is “0,” that is a generic fault that applies to all vehicles. If the first number is “1,” it is specific to the vehicle manufacturer (cheaper code readers may be unable to detect manufacturer-specific codes).
Some trouble codes identify a specific part or location where a problem was detected, such as P0201, for a fuel injector malfunction on cylinder No. 1. Most codes don’t identify a specific part that needs to be repaired or replaced; they only identify a system. P0455, for example, is for the evaporative emission control system, and a loose or worn-out fuel cap, damaged hoses, problems in the fuel tank, or other issues could trigger the code.
The bottom line: An OBD scanner can point you in a general direction, but unless you’re a qualified technician, it can be difficult to pinpoint where the problem lies and how to fix it.
How Much Does It Cost to Diagnose and Fix a Check Engine Light Problem?
Though parts stores will usually read trouble codes with a scanner for free, they don’t fix vehicles. Repair shops charge for this service, ranging from roughly $75 to $150. It typically includes the cost of diagnosing the problem, and most shops use sophisticated (and expensive) scanning tools that can pinpoint trouble areas instead of just pulling trouble codes. The diagnostic charge does not cover the cost of new parts or installation labor. Those costs vary widely based on the price of the parts and how long it takes to complete the work.
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