The Complete Guide To The Check Engine Light

There are many reasons the check engine light can come on. It could be something as simple as a loose fuel cap, or it could be a more serious issue with the engine. Nevertheless, it’s important to never ignore it. Read on for a comprehensive guide to the check engine light.

A check engine light might leave you with a sinking feeling, and a burning hole in your wallet. Although it could mean there is an expensive problem like a catalytic converter issue, it may just be a case of a loose part. It’s worth taking a look at it as soon as possible as small issues can be taken care of before they cause further damage, or even void your car’s next MOT. 

What is the check engine light?

The check engine light, also known as the Malfunction Indicator Lamp, is a signal from the engine control unit (ECU) which indicates that there is a fault with the engine system. There are a number of issues that can make the check engine light come on. Exhaust emissions issues, battery issues or other engine-related issues can all be causes.

If your check engine light does come up it’s worth getting a diagnostic done to pinpoint the exact underlying problem. Diagnostic inspections usually cost around £50 to £80 depending on where you are in the country. Mechanics will have a plug-in diagnostic tool that will display the issue to them. Some rare cars may require a dealer to read off certain fault codes, they will have specialist equipment for those vehicles.

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Remember, a “service required’ light is not to be confused with a “check engine” light, however, these are related. When you see a service light on your dashboard, it means that you need an oil change or other similar maintenance. If the light is ignored, you could end up with a check engine light.

The check engine light will usually come up in orange, yellow or amber colour depending on the manufacturer, and a flashing light usually means there is a more serious issue. A flash or self-test when starting the car isn’t an issue, it’s just a check by the ECU to make sure the system is working as it should

Can I drive with a check engine light on?

You may be able to drive with the check engine light on, depending on the performance of your vehicle and the cause of the illumination. In any case, you should always have your car inspected as soon as the light comes on to avoid irreversible and expensive damage. 

Some cars have a red check engine light for alerting you to stop right away, while a yellow light means the problem should be investigated. A blinking check engine light usually means “stop driving immediately”, while a static check engine light means you need to get it checked. 

If the light comes on while on the road, the important thing is to not panic. First, take a moment to hear or feel for any change in the way the car is driving. Are there any strange noises? If there is anything out of the ordinary then the best thing to do is pull over as soon as possible and it is safe to do so. Start by reducing your speed gradually and avoiding any sudden and erratic movements with the vehicle. 

A few things you can do when the check engine light comes on:

  • Tighten the fuel cap: A loose fuel cap can sometimes be the cause of the check engine light coming on. If this is the cause of it, it will take a couple of trips for the light to go off.
  • Overheating: If overheating has caused the check engine light to pop up, the best thing to do is pull over to a safe space and wait. However, if you see any smoke coming out of the hood then do not drive any further. 
  • Oil change: Alongside the check engine light, if you see the oil level warning sign then you may just need an oil change to fix it. Checking the oil level and refilling it could fix the issue. 

Can I ignore a check engine warning light?

If a solid light comes on means you should get the car checked soon, while a blinking light needs immediate attention.

Remember that the check engine light in all cases means that engine performance is being impacted, hurting your fuel consumption and your wallet as you pay for more fuel. Get the car checked by a mechanic. You should get a quote for the amount to fix the issue, and the amount of time before it causes further damage. This will let you budget the problem before it spirals into a more expensive problem.

Will I pass the MOT with check engine light on?

It’s essential that you pass your MOT, and an engine light illuminated is now a major MOT fail. It will mean your car is no longer roadworthy. The check engine light will sometimes mean there is an emissions problem with your car. The emissions are checked during an MOT so if there is an issue your car will fail its MOT. It’s therefore important to get a check of the engine light and get it fixed so that your car can coast through the MOT without any issues.

Remember, you may not feel immediately something is wrong with the car even if the check engine light is on. Newer cars will alter how the engine works so performance is not severely affected, meaning you may not feel a problem for a long time. However, the issues will be hidden in your fuel efficiency and emission levels. Make sure you never ignore the check engine light and get it looked at before it is too late.

Can I diagnose the check engine light myself?

You could buy your own basic plug-in diagnostic tool which connects to the car’s onboard diagnostics (OBD) port. These cost anywhere from £15-£40 but some older cars may require expensive specialist tools instead. Some diagnostic tools can even switch the light off, which may be tempting to do but keep in mind that it will not make the actual underlying problem go away. Your light will most likely come on again at some point.

A diagnostic tool will show you all the fault codes on your vehicle, but it does not show you exactly which parts need replacing or what repairs are needed. Fault codes will point to a problem area on a car and not an exact repair. Further investigation to check the live data is needed to pinpoint the actual underlying issue. This may entail replacing parts to eliminate any potential causes. This is best left to a professional mechanic who can use his expertise and experience to make the right judgement as to what is needed.

What can I do about the check engine warning light?

Check engine light on dashboard

There are several minor issues that you can solve which may get the light to come off.

1. Check dashboard gauges

These could indicate low oil pressure or overheating and can be fixed once you pull over. You can stop wherever it is safe to do so.

2. Tighten your fuel cap

A loose fuel cap can mean your fuel is getting contaminated by dirt, bugs, or water while driving. Contaminated fuel means that fuel is left unburnt in the exhausts, which triggers emissions warnings. It may take several journeys to reset, and there may be a separate light altogether in your car.

3. Lower your speed & reduce your load

An illuminated check engine light could mean serious problems with your car’s engine. Try to reduce its workload by reducing the speed you go at and, if making a further journey from your destination, remove anything that weighs it down significantly.

What Are The Top Causes For A Check Engine Light?

The check engine light could come from a variety of reasons around the engine, fuel and exhaust systems. A diagnostic tool will tell you which system is at fault, and then you can perform tests to find the exact problem. Here are a few common issues that can cause a check engine light:

A battery fault can cause a check engine light

Check the condition of the battery with a voltmeter, it should at least meet the minimum cranking amperage, as stated in your owner’s manual. Over time, the battery may drain, or after a night with a light left on. Additionally, check for any corrosion of the various terminals and any damage to the fuse box.

Fan, alternator or serpentine belt faults can cause a check engine light

The belts do a lot of work to keep the car cool, generate electrical energy and are an integral part of the engine. They are subject to wear and tear over time or can be damaged by loose debris. You should check the condition of the belts and make replacements to improve the quality and reliability of your engine.

An oxygen sensor fault can cause a check engine light

The oxygen sensor monitors the unburnt oxygen from the exhaust. This also monitors how much fuel is burnt which is directly linked to emissions. The sensors are regularly covered in exhaust fumes which can affect the quality of the sensor. A faulty sensor will mean the ECU adjusts to decrease the efficiency of the engine, meaning a higher MPG. It will eventually lead to catalytic converter problems, which can cost over £2000. Oxygen sensors can be easily replaced, with the DTC telling you which one is faulty.

A faulty fuel cap can cause a check engine light

A loose, cracked, or broken fuel cap will cause contaminants to enter the system or fuel vapours to leak out. These will both have a noticeable impact on emissions, which usually trigger the engine light. To fix it, you should get it tightened, or replaced if it is cracked. A fuel cap shouldn’t cost a lot and all you need is a screwdriver to fix it. This should also help your MPG.

A faulty catalytic converter can cause a check engine light

The catalytic converter is part of the exhaust system, working to reduce the amount of harmful exhaust gases. It converts potentially lethal carbon monoxide into carbon dioxide. A failing catalytic converter will decrease your fuel efficiency and may put the car into limp mode. It usually fails when driving in purely urban environments, with many stop-starts, which deny the catalytic converter from regenerating. You can fix the catalytic converter by taking a half hour continuous drive on a motorway, which will activate systems that clean soot from the catalytic converter. If the problem persists then you will need a mechanic to do a forced regeneration or get an expensive replacement.

A faulty mass airflow sensor can cause a check engine light

The mass airflow sensor sends signals to the ECU about the amount of air coming through to the engine, so the right amount of fuel can also be injected. A faulty mass airflow sensor may have your car stalling and have a noticeable impact on MPG. It can fail because of a faulty air filter, which you should replace once a year in your annual or scheduled service. You can keep driving with a broken mass airflow sensor but you’ll notice the hole in your wallet. It doesn’t cost a lot to replace an air filter, but if the problem persists then you may have to get a mass airflow sensor replaced, which can cost in the region of £200.

Faulty spark plugs can cause a check engine light

The spark plugs and ignition coils provide the spark for every little explosion in your engine. Failing plugs can be noticed by a car ‘hiccupping’ when accelerating. They fail roughly every 30,000 miles and get replaced in our major service.

There is obviously a vast array of reasons, which can leave most people nervous to get a check engine light checked out. You should be responsible for your car and the safety of others on the road. Sometimes it isn’t a long procedure but it’s important you understand what exactly is wrong with your car before it gets worse.