Why Is My Motorcycle Check Engine Light On? A Simple Explanation


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If you pay attention to the dashboard of your motorcycle when you start it up, you will notice a cluster of lights that cycle on for a few seconds and then off for the rest of your ride. If these lights stay on then there is likely a problem with the operation of your motorcycle. While all of the lights on the instrument cluster are important, we will focus on the check engine light for now.

Why is my motorcycle check engine light on? Your motorcycle engine light will turn on due to the sensors in and around the engine detecting a fault in the operation of the engine. Conditions that will cause the check engine light to illuminate are a misfire, lean combustion, rich combustion, O2 sensor issues, and fuel system issues. 

These conditions are important and should be addressed as soon as possible because if they are left to fester, they can become a much more serious and expensive repair. If you are not comfortable working on these issues, you can always call a local shop and see when they are available to take a look at your motorcycle. We’ll discuss further why your check engine light is on and how you can address it properly.

Why Your Motorcycle Check Engine Light Is On

Your motorcycle check engine light is on because something is wrong with the engine. As stated above causes include; misfire, lean combustion, rich combustion, O2 sensor issues, and fuel system issues.

Misfires have three causes: lack of spark, lack of compression, and lack of fuel. These issues are easily diagnosed. If the motorcycle is misfiring you will want to remove the sparkplug and look it over. You need to see if it has carbon build-up on it, if it has uneven wear, or if it has excessive wear.

Carbon build-up will look like a lot of soot that has built up on the spark plug and hardened; this can cause the spark plug to malfunction since carbon can and will divert the electrical spark. If the spark plug is shiny and clean when you pull it out of the engine, and it was not replaced very recently, then the engine should be checked for a coolant leak.

If the tip in the center of the spark plug has worn down, and the spark no longer jumps the gap, it may be time to replace the spark plugs. If you look at the metal ‘j’ shaped piece that hangs below the central electrode, you should see a circle in the center. If this circle is not in the center, then the spark plug is wearing unevenly and should be replaced.

When replacing spark plugs it is important to make certain that the spark plug is the recommended plug and the correct brand. The plug that is recommended by the manufacturer in the owner’s manual is what should be used. The brand matters because the different brands of spark plugs have different heat ranges, which will affect how long the plugs work effectively in the engine.

Before installing the plug make sure that it is the same length as the old plugs and that the spark plug gap is correct. These simple checks will save your engine from breaking itself on a spark plug that is too long.

If the spark plugs and wires are functioning correctly then we can move on to checking compression. Compression tests are easily done if you have or rent a compression testing kit. The kit has a threaded tube that goes into the sparkplug hole (Again be sure that you don’t thread in too long of a hose as it could hit the piston and damage the engine). A normal compression reading should be around 120 PSI; anything under 100 psi could cause the engine to not run. If it’s below 100 PSI I would say it is worth taking into a shop for diagnosis.

Third, if spark and compression check out, it is time to check your fuel. First I would start with the fuel pump, can you hear it run when you turn on the key? If so, you can check for pressure using a fuel pressure testing kit. This should have an adapter that hooks up to your fuel line.

Make sure the fuel pressure is within the specifications for your motorcycle. If it is, then you will want to check fuel flow for three key cycles, you are looking for a good solid flow. If the pump and filter pass all of these tests then you will want to check the fuel injectors (if your motorcycle has them). Check that the fuel injectors have electrical power and ground. If so then you can safely assume that the injector is clogged and you can replace it.

If your motorcycle is carbureted you will want to check that those are not clogged and are working well. If you have let fuel with ethanol sit in the carburetor for a few weeks then you may need to take them out and clean them again. You can learn more about using ethanol in your motorcycle by reading our article “Is Ethanol Bad For Motorcycles? An Engineer’s Explanation.”

If the Motorcycle has lean or rich codes that means that the air and fuel ratio is off. Gasoline engines are made to run at a ratio of 14.7:1. That is 14.7 parts air to 1 part fuel; this allows for complete combustion without excessive pollutants in the exhaust. If your air hoses are broken or cracked then the ratio will be off due to the sensors not reading some of the air. These issues are often easily found with a thorough visual inspection and a smoke test. Since most people don’t have a smoke machine a visual inspection and some hose wiggling will often show you cracks that will allow air to sneak into the system.

How To Read Check Engine Codes On A Motorcycle

If your motorcycle is a 2021 or newer it likely has an OBDII port on it and you can hook up a scanner from any auto parts store to read the codes.

If it is older than 2021 read on.

To check codes on a Harley-Davidson you just need to hold down the trip button that is either on the left handlebar or on the ‘dash’. Then you turn the key on and watch the needles sweep across the speedometer. The motorcycle will then show the word ‘diag’ which means that you are in diagnostic mode. You can then use the trip button to scroll through the modules. It should show a ‘y’ or ‘n’ depending on whether the bike has a code in that module. If you hold the trip button you should be able to see what codes the module has. Here is a good video to walk you through it.

For many other motorcycle manufacturers, you can buy an adapter that will allow you to use an OBDII scanner on your motorcycle. An OBDII scanner and the adapter cords are available on amazon. You’ll want to make sure the cord says that it is compatible with your make before you buy it though. Then you just plug the cord into the diagnostic port on your motorcycle, plug in an OBDII scanner and read the codes off of it. If you just buy the cord, you can always go to any parts store and borrow their OBDII scanner for free.

The scanner will give you a code like P0300 that you can google along with the make and model of your bike to find out what it means, often a likely cause will also pop up in the search results.

How To Clear Check Engine Codes On A Motorcycle

If you use the code scanner you can clear the codes there with the press of a button. With the key on and the engine off, plug in the code scanner, go to read codes, then there should be a button that says clear codes, press it and the codes will be cleared. Sometimes you have to play around a little bit on the code scanner to find the exact page that allows you to clear the codes, but within 5 minutes or so you should be able to figure it out.

For Harley-Davidsons, you will want to go to the trouble code as explained in the section above and push and release the trip button once, then push and hold the trip button. This should make the message change from the code to ‘clear’ which means that the code was successfully cleared.  

Should I Stop Riding Immediately After The Motorcycle Engine Light Turns On?

When the check engine light turns on you should ask yourself, does anything sound wrong? You’ve likely listened to the roar of your motorcycle quite a bit so if something sounds off, you should probably pull over and have it towed into a repair shop or back to your house so you can scan the codes and fix the issue.

If nothing sounds wrong then it is probably safe to keep driving, but make sure to scan the code and fix the issue soon.

Tyler Stott

Tyler is currently studying Automotive Engineering Technology with a minor in Engineering. His interest in cars and motorcycles sparked about 7 years ago and enjoys them so much that he made a career out of it.

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