The check engine light, also called the engine management light, is the yellow engine-shaped warning light, often described as amber or orange. It comes on when the engine control unit detects a fault, usually one that could increase exhaust emissions. What matters most is whether the light stays on or flashes. If it flashes, break off your journey: continuing to drive could damage the catalytic converter. If it stays on and the car drives normally, you can drive carefully to a workshop, but do not put off diagnostics.
Check engine light on or flashing: can you keep driving?
| What you see | What to do |
|---|---|
| The light comes on when you switch on the ignition and goes out after you start the engine | This is the warning light self-test. Everything is normal. |
| The light stays on steadily, the car drives normally and there are no other warnings | The control unit has stored a fault. You can drive carefully to a workshop: book diagnostics within the next few days. |
| The light flashes, but the engine runs smoothly | This usually means misfires that can quickly damage the catalytic converter. Slow down and do not continue your journey: drive carefully straight to the nearest workshop. |
| The light flashes or stays on and the car judders, noticeably loses power, produces smoke or smells of fuel | Stop somewhere safe and call for assistance. |
| A red oil pressure or temperature warning light is on | This is a separate, more urgent warning. Stop somewhere safe and switch off the engine. |
The flashing light is a warning required by regulations: it appears when misfires are frequent enough to risk damaging the catalytic converter. Fuel that has not burnt in the cylinder reaches the exhaust and overheats the catalytic converter.
The check engine light is never red: regulations prohibit that colour for this warning light. It does not change colour as a fault gets worse. Red warnings, such as oil pressure or coolant temperature, are separate lights.
Common causes of a check engine light
- Misfires – worn spark plugs, a faulty ignition coil or an injector.
- Lambda sensor – increased fuel consumption and uneven engine running.
- Catalytic converter – fault code P0420, although it does not always mean the catalytic converter is damaged.
- EGR valve and DPF – in diesel cars, usually with reduced power or limp mode. Read more in our article on the DPF warning light.
- Sensors and leaks – for example, an airflow meter, pressure sensors or a leak in the air intake system.
- LPG system – a poorly adjusted or worn system changes the air–fuel mixture, which the control unit detects.
The light alone does not tell you which fault is present. That takes a diagnostic check.
Check engine light after refuelling: could it be the fuel cap?
It could. Many cars check the fuel system for leaks, and a loose or damaged fuel filler cap creates a leak that the control unit can detect. Check that the cap is properly closed. The light may take several journeys to go out while the control unit confirms that the system is sealed. If it stays on or the engine runs worse, there is another cause.
Check engine, EPC and glow plug lights in VW, Audi, Skoda and SEAT cars
VAG group cars have several engine-related warning lights:
- Check engine (a yellow engine symbol) – a fault that could increase exhaust emissions. It flashes when the engine misfires.
- EPC (yellow EPC lettering) – an electronic engine management fault. The car may limit power.
- Glow plug light (a coil-shaped symbol in a diesel) – comes on briefly when the ignition is switched on while the glow plugs heat up. If it stays on or flashes while driving, it indicates an engine management fault, not worn glow plugs. Drive carefully to a workshop.
Always check your car's handbook for the details. Messages on the dashboard display take precedence over general advice.
A fault code is only a clue: P0300 and P0420 explained
A fault code points to a system or circuit, not a specific part. Two examples:
- P0300 – misfires in multiple cylinders. Spark plugs and ignition coils are possible causes, but so are fuel injection faults, an intake leak, an EGR valve fault or low compression.
- P0420 – catalytic converter efficiency below the threshold. The catalytic converter may be at fault, but an exhaust leak or a faulty lambda sensor can also trigger it. Replacing the catalytic converter on the strength of the code alone may solve nothing.
A basic OBD reader shows emissions-related codes and some engine parameters. This is a useful starting point, not a diagnosis: it will usually not read other control units, such as the ABS or airbag modules, and cannot replace measurements. We read the fault memory of all control units, check live data and take measurements before replacing any part. On newer VAG models, some procedures, such as coding and adaptations, require SFD security unlocking. We use official access for this.
Will a car with the check engine light on pass the Polish roadworthiness inspection?
The Polish roadworthiness inspection (badanie techniczne, often called przegląd techniczny) takes place at an authorised vehicle inspection station (stacja kontroli pojazdów, SKP). The inspector checks that the check engine light works correctly, that the OBD system has completed all its self-tests (known as monitors), and whether any fault codes are stored. In a petrol car, a serious fault reported by OBD is classed as a serious defect (usterka poważna); in Euro 4, 5 and 6 diesels, stored emissions-related fault codes are also classed as serious defects. The car then fails the inspection.
Clearing the codes just before the inspection will not help. It also resets the OBD monitors, and the inspector checks whether they have been completed.
How to turn off the check engine light and why it comes back
The way to turn off the light is to fix the fault. After the repair, we clear the code and check that it does not return.
Simply clearing the code fixes nothing. If the fault remains, the light will return as soon as the control unit detects it again. Clearing the code also deletes the stored data about the conditions in which the fault occurred, which helps identify the cause. Disconnecting the battery to turn off the light has the same effect: it turns off the light but does not fix the fault.
Check engine diagnostics in Kraków
Our auto electrician carries out computer diagnostics at both workshops: in Nowa Huta and Prądnik Czerwony. If the cause is mechanical, such as spark plugs, injectors or leaks, our car mechanic will handle it. We repair lambda sensors, catalytic converters and EGR valves as part of our exhaust repair service.
Check engine light questions
Can the check engine light go out on its own?
Yes, if the fault goes away, for example after tightening the fuel filler cap. The control unit then turns the light off after several journeys without the fault, but the code remains stored in its memory. If the light keeps coming back, the cause is still there.
Does the check engine light mean a serious engine fault?
Not always. A sensor fault or a loose fuel cap can also trigger it. That is no reason to ignore it, though: a minor fault left unresolved can damage expensive components. A worn lambda sensor, for example, can damage the catalytic converter.
Can an LPG system trigger the check engine light?
Yes. A poorly adjusted or worn system changes the air–fuel mixture, and the engine control unit detects it. Tell us the car has LPG when booking diagnostics.
How much does check engine diagnostics cost?
The price of computer diagnostics is in our price list. If a repair is needed, you receive a quote before work begins.
Sources
- UN ECE Regulation No 83, Annex 11 – malfunction indicator lamp (MIL) requirements
- Polish regulation on the scope and procedure of vehicle roadworthiness inspections (Rozporządzenie w sprawie zakresu i sposobu przeprowadzania badań technicznych pojazdów)
- Volkswagen – warning and indicator lights
- Volkswagen UK – emission control warning light
- Volkswagen UK – glow plug and diesel engine management light
- Ross-Tech – fault codes P0300 and P0420
- Ross-Tech – what standard OBD-II can read
- Ross-Tech – SFD security