The check engine light (CEL) coming on can be the stuff of nightmares, as it can signal an impending trip to the shop for potentially expensive repairs. It can be particularly annoying if it comes up after an oil change, as it will leave you second-guessing yourself or your mechanic. Did someone screw up such a basic job?
Changing the oil is a very important maintenance step in ensuring the health of your engine, and shouldn’t normally trigger the CEL. In most cases, a check engine light coming on after an oil change is caused by an oil cap fitted backward, a dipstick that is not pushed all the way into the tube, or overfilled oil.
Fortunately, it’s a relatively common issue and generally doesn’t mean anything serious has happened to the engine. It can be fixed quite easily by simply re-fastening loose components, or siphoning out some oil with a vacuum pump. In this guide, we’ll look at each of these possibilities in depth, and tell you how to clear the check engine light manually or using a diagnostic tool.
What is the Check Engine Light?
The malfunction indicator light (MIL) is a visual sign from your car that something is wrong with the engine. It’s part of your car’s onboard diagnostic system. If the ECU finds a problem, such as a misfire or a faulty sensor, it will relay this information to the driver through the MIL in the vehicle’s dash. The problem can range in severity from a non-issue to potentially serious, and may or may not affect drivability.
The MIL is more commonly known as the “check engine light” or CEL, because in most cases it will display the “check engine” text next to the lamp. It should not be confused with the service reminder light, which on most cars is a separate bulb on the dash.
Luckily, an error code is usually stored whenever the MIL or CEL is illuminated. This can give you a pretty good idea of where the problem lies. The error codes can be accessed with an OBD2 scanner in vehicles made after 1996. Pre-OBD2 vehicles may have other devices for reading error codes, such as a Morse-code-style display with blinking lights under the hood.
7 Reasons Your Check Engine Light Came On After an Oil Change
After changing the oil in your car, you might find that the check engine light comes on, which can be frustrating. Here are a few common oil change mistakes that can trigger the check engine light:
1. Loose or misplaced oil cap
This is the most likely reason for a CEL coming on after an engine oil change. It’s possible to install the oil cap backwards or forget to fit it entirely, especially for a busy workshop trying to rush an oil change job. The good news is that no permanent damage is likely to result from this, so you can simply refit the gas cap and be on your merry way.
You might be wondering how something as simple as a loose or misfitted oil cap could cause the check engine light to come on. To understand the answer, you must first understand how the engine works.
Put simply, a conventional gasoline engine works at an air-to-fuel ratio (AFR) of around 14.7-to-1. This means the engine control unit (ECU) needs to supply the engine with 1 gram of fuel for every 14.7 grams of air that finds its way to the combustion chamber.
This is called the stoichiometric air/fuel ratio, which is the theoretical point where the chemical reaction of combustion is complete: gasoline and air (oxygen and nitrogen) are converted into carbon dioxide and water vapor. In an engine, this means the best balance between power delivery, fuel efficiency, and lower harmful emissions.
To know how much fuel it needs to inject, the ECU (also known as the engine control module or ECM) needs to know the exact amount of air entering the engine at any moment. This is usually measured by a mass airflow or MAF sensor.
This sensor is mounted on the intake air pipe, just downstream from the airbox, and precisely calculates the intake air mass. The ECM then uses this information to determine how long it should open the fuel injectors, feeding the engine with just the right amount of fuel for the right amount of air.
This is where your loose oil cap comes in. A badly fitted oil cap will introduce unmetered air into the engine through the valve cover, and then, the intake valves. This means air comes into the engine downstream from the MAF sensor and, therefore, is unaccounted for. This will cause the mixture to turn lean, that is, there is more air in the cylinder than what’s needed for stoichiometric combustion.
In modern cars with fuel injection, the computer has an extra ace up its sleeve: the oxygen sensor or lambda sensor. This sensor is mounted on the exhaust pipe and measures how much unburned oxygen leaves the engine. The ECU uses this information to determine how the mixture actually is – too little oxygen and it’s too rich (i.e. too much fuel for the amount of air), too much oxygen and it’s too lean.
This allows the ECM to compensate for the extra air of vacuum leaks (such as a loose oil cap) with extra fuel, maintaining the optimal air-fuel ratio. So, you may not notice any drivability issues.
What about the CEL, then? Well, with the information from the MAF and other sensors (such as the throttle position sensor), the ECM knows how much air should be entering the engine. If it has to compensate, then it knows something is off, and might trigger the check engine light to let you know about this.
2. Loose or misplaced dipstick
A loose lipstick can also cause a check engine light much in the same way as a loose oil cap: by allowing air into the engine that wasn’t accounted for. In this case, the extra air can come into the combustion chambers via the PCV system or directly through the piston rings during the intake stroke.
3. Overfilled oil
It is very easy to put in a little more oil than you should when filling up your car after an oil change. Just a little above the “max” level on the dipstick won’t cause any harm, but severely overfilling your engine with oil can lead to some pretty bad consequences.
The excess oil will increase the oil pressure, which may force oil into the intake system, causing it to find its way into the combustion chamber. This will foul spark plugs and cause engine misfires which will trigger a CEL and potentially damage the catalytic converter.
The extra oil pressure will also put extra strain on seals which may spring a leak as a result. Furthermore, it can cause the crankshaft to foam the oil in the crankcase by directly coming in contact with it. This greatly reduces the oil’s lubricating properties.
4. Low oil level or wrong oil grade
It’s debatable whether having a low oil level or the wrong oil grade can trigger a check engine light. While neither condition will cause the CEL to light up per se, a low oil pressure condition may affect some of the car’s systems which could. For instance, cars equipped with variable valve timing control that depends on oil pressure to operate might throw a CEL with a camshaft timing error code.
When changing the oil, always remember that you have to check the oil level after starting the car and letting it run for a few seconds, so the system is pressurized and the filter is filled with oil. This will cause the level to drop, and you may need to top it up again.
5. The service light didn’t reset
Most modern cars have a separate light to let you know when it’s due for servicing, but in some cases the check engine light doubles as the service reminder. If that’s the case with your car, then it might simply need resetting after the service is done.
6. Unplugged or dirty sensors
We’re getting into the more left-field problems here, but something as simple as an unplugged or dirty sensor might be causing the CEL. If you had to disconnect any sensors or wiring on your way to the oil filter, make sure you remembered to plug them back in. Also, if you spilled oil somewhere outside the engine, it might be a good idea to get that cleaned up.
7. Other unrelated reasons
If all else fails, you should consider the possibility that the CEL is on by coincidence and unrelated to the oil change you just performed. Before anything else, check that your gas cap is well tightened, as that is a very common (and often overlooked) cause of a check engine light coming on. Otherwise, read the error codes with a diagnostic tool – that will show you where to look.
4 Steps to Diagnosing Check Engine Lights After an Oil Change
Now you know the possible causes of a CEL coming on after an oil change. What do you do next? Follow these steps to find out how to diagnose it.
1. Check the oil cap and dipstick
The first thing you should do if a CEL turns on after an oil change is to check the oil cap and dipstick. If any of these is loose or incorrectly installed, it could cause the CEL to turn on.Make sure the oil cap is not on the other way around, and that the dipstick is pushed all the way in.
Take the opportunity to check the seal around your oil cap, as these can deteriorate with age and compromise the cap’s ability to seal properly. It’s a good idea to replace the seal every few oil changes to be on the safe side. If it’s dry or cracked, replace it now.
2. Check the oil level
The second thing you should do is check the oil level. With the vehicle parked on level ground, pull off the dipstick and clean it with a lint-free rag. Replace the dipstick, pull it off again, and read the oil level.
It should be between the “min” and “max” marks on the dipstick. If it’s below the “min” mark, then the level is too low and you should top it off with the correct oil for your engine. If it’s above the “max” mark, then the oil is overfilled and you should remove some of the excess or perform a fresh oil change.
3. Scan for error codes
If everything seems fine and the CEL keeps coming back, your last resort is to pull up the diagnostic tool and check the error codes for a clue as to what the problem may be. Find the OBD2 port in your car (it will usually be located below the steering column) and hook up your diagnostic tool. Read the error codes and look them up to find out what exactly is wrong.
How Do You Fix a Check Engine Light After an Oil Change?
Now that you have identified the source of the check engine light, removing it should be relatively simple. Here’s how:
1. Place the oil cap or dipstick correctly
If your CEL was caused by a loose or incorrectly fitted oil cap or dipstick, simply replace them. Make sure the cap is installed the right way – this will normally be with the “oil” lettering facing the front of the car. The dipstick should be pushed all the way in, so that the O-ring can seat and seal properly.
2. Remove some overfilled oil
If the problem is that you filled up with too much oil, you may get away with simply removing some of it, as long as you haven’t driven the car yet.The best way to remove some excess oil is to siphon it through the dipstick.
This can be accomplished with a simple hand-operated vacuum pump, or an electric pump. The latter can be a good investment if you want to be able to change your oil without making too big of a mess. You may even be able to do the job without lifting the car up, provided your oil filter is accessible from above).
3. Top off the oil
If the oil level was very low, then you should fill it up. It’s that simple. Just make sure to use an oil with the correct viscosity, and which meets the standards set by your car’s manufacturer. Your owner’s manual will contain all this information, so you can make the right decision when purchasing the oil.
4. Reset the check engine light
If you made sure the oil cap and dipstick are nice and tight, and the oil level appears to be correct, then maybe the light wasn’t reset entirely. So, your next port of call could be to reset the CEL and see if it comes back up again. If you have a post-1996 car, you can do this by using an OBD2 diagnostic tool.Here’s the step-by-step guide:
In most cases, this can also be done manually.This will generally involve turning the ignition to the on and off position several times, but the exact procedure will vary from car to car, so look up how to do it for your specific vehicle.
Simply disconnecting the battery’s negative lead for a few seconds and putting it back again might do the trick, but make sure you write down the radio code if you have it. It might also make your car run poorly if the CEL was triggered by vacuum leaks that the engine was compensating for, as the fuel trims will be reset by doing this.
FAQs
Will the check engine light reset itself?
The short answer is: it depends. Sometimes the light will reset itself once the underlying issue is fixed and after a few cycles of the engine being turned on and off. If you are sure the cause for the CEL is fixed, you can wait a few days to see if the light clears itself, however your best bet is to reset it manually or with a diagnostic tool.
Does check engine light mean low oil?
No, the check engine light doesn’t necessarily mean your oil level is low. It means there’s a problem with some of the systems in your engine. Normally this will be a misfire or a faulty sensor. The low oil level will cause low oil pressure which may cause the red oil pressure light to illuminate in the dash. It can also trigger a CEL if one of the engine’s systems can’t operate well due to low oil pressure (for instance the variable cam timing).
How do I clear my check engine light without a scanner?
The easiest way to do this is to perform an ECU hard reset by disconnecting the negative battery terminal,then pressing the brake pedal for a few seconds to discharge any lingering voltage on the system. There are a few downsides to this, so only do it if you are sure your engine is working well and you have no other option.
Written By
Victor Faeda
Victor is a Mechanical Engineer BSc and certified Automotive Mechatronics technician from Portugal. He worked as an intern mechanic for Volvo and has been fixing his own vehicles for over 10 years. Writing and cars are his passion, so now he combines the two by creating content around the automotive industry. He specializes in automotive technology and maintenance.