common-ignition-problems-870x489The Check Engine Light (CEL) is never good news when it comes on in your car. At a minimum, something as simple as your gas cap may be loose or it could be a failed sensor, wiring, connector. Worse yet, it could be a larger problem like an expensive emissions fault. Ignoring it if the car runs and drives fine may seem reasonable for some time, but problems will continue to mount. Here at Partste, we’re pretty familiar with the light, so we’ve rounded up the 5 most common causes and everything else you need to know when you’re wondering, “Why is my Check Engine Light on?”


If the light is steadily lit, that means that one of your engine sensors is doing one of two things – it’s either sending a signal to the vehicle’s ECU that something is wrong, or the vehicle’s ECU (computer) itself has lost an accurate read on that particular sensor – meaning, the sensor has gone out. Sometimes, multiple sensors send signals, indicating clues that something bigger has gone awry. You can still drive with the light illuminated, assuming the car is running and driving fine, but you should get it checked out as soon as possible. If the light flashes, that means you need to pull over immediately as your vehicle has a serious issue that could cause imminent damage.


You can. All that you need to check your own light is an OBD-II reader, which can pull codes that help you find out what’s wrong with your car. Reasons for Check Engine Light illumination are vast with literally hundreds of potential codes in the onboard computers. If you don’t have a code reader, keep reading to learn the most common causes, and then bring your car to your local AutoZone to find out why your Check Engine Light is on.




An automotive professional shop invests thousands of dollars in diagnostic equipment and training for said An automotive professional shop invests thousands of dollars in diagnostic equipment and training for said equipment. While yes, both yourself and Partste can read the engine computer’s stored codes, and through Fix Finder, determine likely causes based on thousands of mechanics intel, none of these can pinpoint with 100% accuracy exactly why the CEL came on.

Many times, multiple sensors are telling us something but not telling us why, and in nearly all these cases, the vehicle needs to go to a mechanic where more specialized diagnostic equipment can diagnose the vehicle while the car is running. They can then determine exactly what is going on beyond just trouble codes. A code reader and an ASE-certified Master Tech with thousands of dollars in diagnostic equipment are NOT the same equivalent at all.

If there is any speculation or doubt about exactly what’s going on with the trouble codes retrieved, you can search here for Partste preferred repair shops in your area that can help take the information you already have and assist you in the repair. It’s important to note that some trouble codes can and do lead to an easy result and solution. Many cannot, and even the most avid of DIYers need the assistance of professional diagnostics.



Your oxygen sensor measures the amount of unburned oxygen in your vehicle’s exhaust system. It indicates how thoroughly the air-fuel mixture burned during the combustion process.

Oxygen sensors also live a hard life in +800 degree exhaust temps and are prone to fail over 80,000 miles or more. It’s important to note here that there are a multitude of engine codes where the oxygen sensor indicates something is wrong, not necessarily that the oxygen sensor is bad. Be careful determining that a sensor is truly at fault, as opposed to determining something else is happening. This cannot be more apparent than in the ever-popular PO420 / PO421 engine code.

What happens if I don’t replace it?

Your engine will burn more fuel than needed
You will lose fuel economy, and get fewer miles per gallon
Faulty sensor(s) can cause damage to your spark plugs and catalytic converter


Your gas cap is part of a sealed evaporative emissions system that recirculates gasoline vapors from your gas tank and keeps them from escaping into the air. Because of this, this system of lines and valves is closely monitored, and one of the key sealing pieces of this is the gas cap.

If you leave your gas cap off accidentally, it won’t take long before the diagnostic system sets Check Engine Light codes. These leaks in the EVAP emission control system are often grouped as “small” or “large” leaks. Its important to note that the problem is not always the gas cap, but could be one of the plastic recirculation lines, fittings, or connectors causing the leak, but more often than not, the gas cap is the culprit.

What happens if I don’t replace it?

You can lose fuel through evaporation
Your recirculation system will not function properly
You will have to stare at the CEL until it’s fixed


Your catalytic converter helps protect our environment by superheating hydrocarbons and other harmful tailpipe emissions into compounds like carbon dioxide and water vapor. Your downstream oxygen sensors carefully monitor the converter’s performance, and if something goes awry, a number of trouble codes based on “Catalyst efficiency” will appear.

It’s important to note that a catalytic converter will more than likely not fail on its own. Generally, something causes it to fail or not perform properly. Because of this, simply replacing the converter or the oxygen sensors around it will often not fix the problem, and often leads to yet another failed converter. Things as serious as a blown head gasket can force burned coolant vapor into the exhaust, triggering a catalyst engine code.

What happens if I don’t replace it?

Your vehicle will not pass an emissions test
You will experience reduced performance and fuel economy
Whatever caused the converter to fail initially will continue to get worse, possibly causing engine failure


Your mass airflow sensor (MAF) measures the amount of air entering the engine to determine how much fuel is needed to run your engine properly. If there are any leaks before or after the mass airflow sensor in the intake tract, a light can trigger. Mass airflow sensors are sensitive to oil, dirt, and water vapor, and any contamination on them can trigger a light. Sometimes, cleaning the sensor with intake or mass airflow sensor cleaner can fix the issue. Other times, the sensor needs to simply be replaced. Be sure to check the intake ducting carefully for leaks, rips, or damage.

What happens if I don’t replace it?

A faulty MAF sensor can cause rich or lean conditions, which can lead to further failures of other parts
You will experience reduced performance and fuel economy


If your car is misfiring, you probably aren’t wondering to yourself “Why is my check engine light on?” Your spark plugs ignite the air/fuel mixture in the combustion chamber of your vehicle, but if the timing of that spark, the mixture of the fuel and compression of the mixture isn’t perfect, you will get a misfire. Multiple issues can cause a misfire, either on one cylinder or a multiple cylinder misfire.

Defective ignition coil:(either DIS or coil-on-plug). A quick trick to determine if a coil is causing a misfire is to simply move the coil and swap with another on the engine. If you have a #2 misfire and move the #2 coil to cylinder #4, for example, then the misfire moves to #4, you know the coil is suspect.
Defective fuel injector: If a fuel injector fails to deliver the proper amount of fuel to a cylinder, you will have a misfire. Unlike swapping the ignition coil with a neighboring coil, a fuel injector is more difficult to move.
Vacuum leak: Vacuum leaks often cause multiple cylinder misfires, or lean running conditions. They also can be very difficult to diagnose.
Worn spark plugs and/or wires: Sometimes, spark plugs can go far past their life cycle and fail, causing a misfire. On vehicles with spark plug wires, a defective wire or wires can also cause this. For many drivers with older cars, it’s a likely reason the Check Engine Light came on.
Bad compression: A burnt or bent valve, worn piston rings, or valve timing that is off will cause a misfire as well. These issues are more serious and will all become apparent during an engine compression test.

What happens if I don’t replace it?

You will experience poor performance (reduced power, engine missing) and reduced fuel economy
The misfire will only get worse, and will lead to mechanical failure in the engine

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