The check engine light coming on can be a scary prospect for most drivers, but what does it mean? It could be a tiny glitch or something serious like a failing part that might lead to a replacement engine. Either way, don’t ignore it. The best advice is to get it checked as soon as possible.
The check engine light is your car’s way of saying something’s not right. It could be anything from battery problems to a bigger engine issue.
Sometimes, it’ll flash or come on briefly and go off again. Other times, it stays solid on your dashboard. It might even appear when the engine is idling roughly.
Whatever your check engine light is doing, you should pay attention – it’s a warning sign that something needs fixing.
Why is my check engine light flashing or coming on and off?
How your check engine light appears can sometimes be a clue to what’s wrong under the bonnet:
A flashing light usually means a serious problem, like an engine misfire, which can damage the catalytic converter. Stop driving and get it checked immediately.
A light that comes on, then off, may just be a temporary glitch or a minor issue, but it still needs diagnosing.
A light that appears during rough idling can be a sign of engine running problems, so it needs addressing to avoid more serious issues.
Common causes of the check engine light
Oxygen sensor failure
The oxygen sensor plays a crucial role in monitoring the amount of oxygen in your exhaust gases. It helps the engine control unit (ECU) adjust the air-fuel mixture for efficient combustion. When this sensor fails, the ECU may not properly regulate the mixture, leading to poor fuel economy and increased emissions.
A faulty oxygen sensor can cause rough engine running and eventually damage the catalytic converter. But replacing it is generally straightforward and can restore your car’s performance and reduce emissions.
Catalytic converter issues or theft
The catalytic converter is essential for reducing harmful emissions by converting exhaust gases into less harmful substances. Problems with this part, such as clogging, damage, or theft (which is becoming increasingly common) will almost always trigger the check engine light.
A failing catalytic converter can cause poor engine performance, increased emissions, and a rotten egg smell from the exhaust.
Mass airflow sensor failure
Your mass airflow sensor measures the amount of air entering the engine to ensure the right fuel-to-air ratio. If it gets dirty, damaged, or fails altogether, your engine may run roughly or hesitate, causing the check engine light to come on.
A malfunctioning mass airflow sensor can also reduce fuel efficiency and cause your car to stall or hesitate under acceleration. Cleaning or replacing the sensor will usually solve these issues and get your engine running smoothly again.
Battery problems
A weak or failing battery can cause all kinds of electrical gremlins, including triggering warning lights on your dashboard like the check engine light. Voltage drops or problems with your power supply can also affect the car’s sensors and electronic control systems.
If you notice dim lights, slow starting, or other electrical issues along with the check engine light, try testing your battery and charging system. Replacing an old battery is usually a simple and cheap fix.
Engine oil or dipstick issues
Low engine oil levels or a loose dipstick can also cause the check engine light to come on. The engine relies on sufficient oil to lubricate moving parts and prevent damage. If oil levels drop too low, sensors detect it and trigger a warning.
Sometimes, simply making sure the dipstick is seated correctly or topping up the oil will solve the problem. However, if the oil light stays on, check for any leaks from the sump or engine wear before it leads to more serious damage.
Spark plugs and ignition coil problems
Worn spark plugs or a failing ignition coil can cause your engine to misfire, resulting in rough running, reduced power, and higher emissions. Misfiring often causes the check engine light to come on.
Replacing spark plugs and ignition coils when you’re supposed to will keep your engine running smoothly.
Vacuum leak
A vacuum leak happens when there’s unintended air entering the engine’s intake system, disrupting the fuel-air mix. It can cause rough idling, stalling, or poor acceleration — all of which can trigger the check engine light.
Cracked hoses or loose connections can cause vacuum leaks. Finding and fixing the leak usually requires a mechanic, but solving it will improve your engine’s performance and fuel economy.
Exhaust gas recirculation (EGR) valve failure
The EGR valve helps reduce nitrogen oxide emissions by recirculating some exhaust gases back into the engine. When it fails or gets clogged, it can cause rough running, increased emissions, and trigger the check engine light.
A faulty EGR valve may also cause the engine to stall or hesitate on acceleration. Cleaning or replacing the valve usually resolves any issues.
Alarm system issues
Sometimes, aftermarket alarm systems or immobilisers can interfere with your vehicle’s electronics, triggering the check engine light. Wiring faults or compatibility problems can cause false warnings or actual engine management faults.
If you’ve recently installed an alarm or security system and the light came on soon after, it’s worth checking with the installer or a mechanic.
Is it safe to drive with the check engine light on?
Whether it’s safe to keep driving when your check engine light comes on depends on the situation. However, our expert advice is to get it checked out, whether it’s steady or flashing. While there’s no fixed time for how long you can drive with the light on, it’s always a good idea not to delay getting it diagnosed.
What to do if your check engine light comes on
If your check engine light turns on, pay attention to any unusual smells, noises, or performance issues that may also be affecting your vehicle, then take it to a garage for an expert check of engine codes. Remember, ignoring the light won’t make it go away!
How will a pro check engine codes?
A professional can use an OBD-II scanner to read the error codes from your car’s computer, giving them a clearer idea of what’s wrong. These devices plug into your car’s diagnostic port, usually under the dashboard. Once connected, they can retrieve error codes that point to specific issues. For example, a code like P0300 means your engine is misfiring randomly. Your mechanic should be able to work out what’s going wrong from the engine codes.
Does a check engine light fail an MOT?
Yes, having the check engine light on during an MOT test is usually a fail. The test looks for any warning lights that are a sign of faults in your vehicle’s systems.
A lit check engine light means your car’s computer has detected a problem that could affect emissions, safety, or performance. So, it’s important to fix any issues and clear the warning before your MOT to avoid failure and keep your car on the road.
What about repair costs?
The cost to fix a check engine light issue can depend on the cause, so costs can vary widely. A diagnostic check usually costs somewhere in the region of £60 to £70, while simple fixes like replacing spark plugs are usually fairly cheap.
However, if the diagnostics throw up a more serious problem involving sensors such as the oxygen or mass airflow sensor, or parts like the catalytic converter, it can be much more costly. Ignoring the warning light might lead to severe damage, potentially requiring a replacement engine, which is a major expense, so get yours checked out sooner, rather than later.
Disclaimer: These articles are for guidance purposes only. If you have any questions regarding any matter
relating to your vehicle we would recommend that you seek the advice of an appropriate professional.
We accept no responsibility or liability should you suffer financial or personal damages in
relation to the advice stated on this website.
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