Check engine light is on-understand how the system works

In order to find and solve why the check engine light is on, you must first understand how the whole system works.

You see, this system of warning was conceived to warn the driver on time if something was wrong with the car management system (engine, electronics, exhaust, and other). Through sensors located on crucial parts of the system, information is given if something is wrong with the car.

This system was meant to replace the old school diagnostic system which relied exclusively on the knowledge, experience, and expertise of a mechanic.

So, when the check engine light comes up, you or the mechanic hook up the OBD II reader through the OBD II port, read the code, replace the part, reset the check engine light, and presto: everything is fixed.

It was a good idea that was supposed to make drivers and mechanics life easier.

But, over time and through exploitation, this system has shown both its good and bad sides.

Good sides and bad sides-avoid the check engine “wild goose chase”

Good side: warning on time and prevention.

Bad side: the main problem of this system is that a faulty sensor, broken wire, short-circuit can send you on a wild goose chase.

Wild goose chase meaning lost money and time on unneeded repairs.

Many times people have ended up paying a ton of money to solve this problem and yet still the check engine light was on. To make things worse, even the mechanics shrugged their heads telling you:” just keep driving like that, nothing will happen”.

Now, we come to the conclusion: the system itself can be helpful but a lot of times it just brings headaches to the driver and car owner.

This article will, hopefully, help you in avoiding these headaches. Through a list of the most common reasons why the check engine light is on you’ll probably get a good lead on your problem if not solve it.

At the end of this article, you can also read about how long you can drive with a check engine light on and the most easiest and effective ways to reset (turn off) the check engine light.

Loose or faulty gas tank cap

loose gas tank cap-check engine light is on

Loose or faulty gas tank caps can be the case.

But….although this is a very popular check everyone makes first, in my experience it is rarely the solution. The gas tank cap mostly gets loose when refueling due to a lack of tightening. When the gas tank cap is faulty it’s mostly because of the rubber seal on the cap. Fuel (petrol) is an aggressive substance which can cause, over time, for the rubber seal on the cap to get stiff and rigid.

In both cases, the problem is that excessive air gets in the gas tank (then in the intake) thus causing the check engine light to go on.

In any case, make this check first. It’s the easiest of all and if it works, good for you.

Oxygen sensor is faulty (aka O2 sensor or lambda sensor)

oxygen sensor-check engine light is on

Faulty oxygen sensors are also common. Since they’re mounted on the exhaust system, over time they get dirty or just break down due to heat and material fatigue.

The OBD reader is the best way to locate this fault (the reader will show the proper code).

The oxygen sensor is located on the exhaust manifold and exhaust pipes and it’s common to have at least one, in more cases, two sensors.

Besides the sensor malfunction itself, the oxygen sensor will react to poor quality fuel, problems with air intake, or anything that can disturb the proper air-fuel mixture.

The replacement is not that complicated. It’s a simple screw-out-screw-in job. So, if you can reach it, it’s pretty much a DIY job. The bad part is that the oxygen sensor is mostly hard to screw off. Due to heat and rust, it can seem welded to the pipe.

If you decide to do this on your own, make sure that the car is stone cold so you don’t burn yourself and that you have the proper tools.

There is also the option of cleaning the old oxygen sensor. This repair, in my opinion, has a questionable outcome.

Bad catalytic converter

This is closely connected to the oxygen sensor fault since a bad catalytic converter will trigger the oxygen sensor which will turn the check engine light on.

Catalytic converters are a part of the exhaust system that cleans and purifies the exhaust fumes.

Over time, the catalytic converter gets dirty and filled up with grit from the exhaust fumes. When this happens the check engine light will go on. This fault usually happens to cars with high mileage or due to excessive use of low-quality gas (petrol).

The solution is to replace the whole catalytic converter with a new one. Since this happens mostly to older cars, people avoid changing the catalytic converter as this is mostly an expensive repair.

There are some alternative (read shady) solutions that are in the DIY realm like taking off the catalytic converter and rinsing it or cutting out the converter altogether and welding a piece of pipe instead.

No need to talk about these solutions. If you mean good to your car, environment, and yourself, just don’t do it.

Faulty mass airflow sensor (MAF sensor)

mass air flow sensor-check engine light is on

This one is also fairly common for why the check engine light is on. Using the OBD tool you’ll get the fault code and save yourself some wandering around.

It’s best to buy and replace it with a new one although cleaning this sensor is also a common practice.

Over time, the sensor gets dirty and clogged thus giving bad readings to the ECU unit which then affects the engine management system.

There are a lot of treatments for this (sprays, cleaners, or else), that can clean the MAF sensor very well.

If you decide to make a replacement, it’s not that complicated in most cars. It’s located on the intake manifold, usually in an accessible place. It can be a DIY job if you have at least some skill in car mechanics.

Air intake problems

road trip with kids-check the

One of the most common causes of the check engine light flashing or turning on is due to a fault in the air intake system.

A loose hose clamp, broken or damaged seal, punctured hose, or else can cause the check engine light to flash. Just like with electric or electronic problems, this fault is very likely to send you on a wild goose chase. Also is known to produce error codes that lead to unneeded part replacement.

For example, a small, barely visible puncture somewhere on the air intake hose can end up in costly repair which will not solve your problem.

Checking the air intake is simple and definitely a DIY job. Just have patience and a good eye and check all the air intake parts (hoses, clamps, seals, housings, etc).

I myself have had a situation like this one and solved it with a simple inspection and with no money. You can read this article or take a look at the video on our YouTube channel.


Car battery condition 


car battery-check engine light is on

Most modern cars are highly dependant on proper battery conditions. The moment the car battery starts to lose its power  a variety of problems can come up.

You see, the ECU is very sensitive to a variable power supply and can turn the check engine light on although everything is OK with the car. Besides this, other weird signs will appear like glitches or warning lights, for no apparent reason.

The solution, of course, is a battery replacement or trying to recharge the battery.

Alternator voltage output  problems

alternator voltage output

Alternator’s main task is to supply the car installation with electricity. If the voltage is too high or too low, it will cause, (just like in the case of a bad battery) the ECU to go haywire.

This goes double if the voltage is too high. You’ll also get glitches, warning lights for no apparent reason, and other electronic problems.

This should be solved as soon as possible as it can cause major damage to the car installation and devices. This can be prevented by checking the alternator voltage output.

Worn out spark plugs (petrol engines)

Changing the spark plugs is part of regular maintenance. They will cause problems if they are worn out or damaged (contacts, insulators, or else). Worn out spark plugs will cause the engine to misfire and work improperly and this is also a reason why the check engine light is on.

Change the spark plugs on time, use quality spark plugs and you’ll certainly evade this problem.

Damaged spark plug cables (petrol engines)

spark plug cables-check engine light is on

Unlike the spark plugs, the cables that supply the spark plugs with electricity are more prone to damage.

Over time, due to heat, vibration, high voltage, or just simple material fatigue, the wires inside the cable break or burn out. This will cause misfires in the engine and why the check engine light is on.

 If this happens, it would be best to change the whole set of cables. If not, you can check the cables for physical damage and that they have good contact with the spark plug.

In any case, it’s pretty much a DIY job, just make sure that you don’t mix up the cables and return in proper order.

Electric or electronic problems

ecu unit-check engine light is on

Short circuits, damaged wire insulation, moisture near contacts are just some of the causes why the check engine light is on.

These types of problems are one of the most complicated to find and solve.

For instance, the OBD diagnostics, in this case, will often read the wrong fault code. To clarify, due to a simple bad contact somewhere in the installation, you could end up changing lots of parts and spending money for nothing.

The only remedy to this problem is to have patience and make a thorough inspection of the whole installation. Look for tell-tale signs of problems (leaks or moisture near the car installation-ECU, fuse box, wiring looms, etc., bad or corroded contacts, tight places that can cause damage to the wire insulation, etc).

How long can you drive with your check engine light on?

Unlike the other warning lights (overheating, low oil, etc.) the check engine warning (in most cases) doesn’t mean immediate danger and hazard. How many times have you seen people driving with this light on for months?

My advice to you is, when the check engine light is on, make a stop as soon as possible and make a quick inspection under the hood. If there’s no big irregularity and you see that you won’t be in danger, continue your trip. Of course, find a mechanic as soon as possible and make a thorough check-up.

The bottom line, if there’s no radical change with the car (engine noises, squeaks, poor acceleration, or else), you can continue driving. The check engine light will often start flashing and you can continue your drive without a problem.

For instance, a minor puncture in an air intake hose is hardly a good reason to pull over and call road assistance. But its reason enough to light up the check engine light.

How do you turn off (reset) the check engine light?

There are a few ways you can turn off (reset) the check engine light :

1. Use an OBD code reader

This is the most common way. Even the cheapest of the code readers can erase the check engine fault. Just connect it to the car via the OBD port, find the reset in the menu, and your good to go.

2. Take the battery terminal off

The most common and simple way to reset the check engine light.

Take off the battery terminal of the battery pole (best you take off the “-“ battery terminal). This way you leave the car ECU (computer) without power forcing it to reset. Helps in lots of cases, just make sure that you take necessary precautions before removing the battery terminal (keep the doors or window open, take the keys out of the car, have at hand the radio code, etc).

Also, I don’t recommend this solution with high-tech cars (with lots of electronics and gadgets) because you can possibly cause even more problems.

3. Repair the cause of why the check engine light is on

This is by far the best solution. Once you’ve found the fault (with the OBD reader or manually) and made the repair, in most cars the ECU will turn off the check engine light automatically.

Just have in mind that it may take some time for the ECU to turn off the light. So, even if you’ve done the repair properly, the check engine light may stay on for a couple of starts (ignitions).

On some cars, you’ll have to use the OBD reader to clear the check engine light.