In brief
A steady yellow check engine light means the control unit has stored a fault. Usually, you can drive carefully to a workshop. A flashing light means a serious misfire: slow down and stop so the catalytic converter isn’t damaged. Reading the fault code reveals the cause in about half an hour.
The check engine light is perhaps the most frustrating of the dashboard warning lights. It tells you something is wrong, but not what. The cause could be a loose gas cap or a failing catalytic converter. The good news: the car knows what’s going on. The fault code is stored in the control unit’s memory.
Is the light steady or flashing?
This is the most important single thing. The status of the light tells you how urgent it is:
- The light is steady: a fault has been stored, but the situation is usually not acute. You can drive calmly, but book an inspection soon.
- The light is flashing: the engine is misfiring. Unburned fuel enters the exhaust system and can destroy the catalytic converter quickly. Slow down, avoid accelerating, and stop the car when it is safe to do so.
- The light is on and the car is running strangely, lacks power, or other warning lights are on: do not continue your journey far.
Most common causes
The same light can come on for dozens of different reasons. The most common are:
- Oxygen sensor (lambda sensor), which measures the oxygen content of the exhaust gas
- Spark plugs or ignition coils causing misfires
- Mass airflow sensor or an intake leak
- EGR valve or diesel particulate filter
- A loose or leaking gas cap, which shows up as an evaporative system fault
- A weak battery, which can cause random fault codes in various systems
How is a fault code read?
Cars have a standard diagnostic port, usually under the steering wheel. It has been mandatory in the EU for gasoline cars since 2001 and for diesels since 2004. A scanner is connected to the port, and the control unit provides the codes it has stored.
A cheap scanner shows the code, but not always why it occurred. For example, an oxygen sensor code could be caused by the sensor itself, an intake leak, or the fuel supply. That’s why the code is just the beginning: diagnostics determine which part actually needs to be replaced. Otherwise, the wrong part is easily replaced.
Can the light just be reset?
Yes, but the fault won’t go away. If the cause still exists, the light will come back on. Additionally, inspections check for emission-related faults in the fault memory, and a recently cleared memory can show up there too. So, find the cause before resetting.
Frequently Asked Questions
Can you drive the car when the check engine light is on?
If the light is steady and the car is operating normally, you can usually drive calmly to a workshop. If the light is flashing, slow down and stop the car as soon as possible.
What does a flashing check engine light mean?
The engine is misfiring. Unburned fuel can damage the catalytic converter quickly, so you should not continue driving.
How much does it cost to read fault codes?
At BX-Motors, reading fault codes costs 25 euros and takes about 30 minutes.
Will the car pass inspection if the check engine light is on?
A lit check engine light and emission-related fault codes can lead to failure. Find and fix the fault before the inspection to avoid a re-inspection.
Does the check engine light go off by itself?
Sometimes, if the fault was temporary. The code will still remain in the memory, and a recurring fault will turn the light back on.
Read also: Particulate filter clogged? · Getting your car ready for inspection · Car won’t start?
