Check Engine light is on: what it means and what to do

The Check Engine light (a yellow engine outline) means the engine control unit has detected a fault and stored a code in its memory. The cause can be minor — a loose fuel cap or a sensor — or it can point to a problem with ignition, boost or the air-fuel mixture. A fault code shows where to look but doesn’t always name the exact part, so the data needs to be analysed, not just read.

How it shows up

• The light stays on while the car drives as usual
• The light is flashing, often with the engine shaking
• The engine has lost power or gone into limp mode
• Rough idle, misfiring, hard starting
• Higher fuel consumption or a fuel smell from the exhaust

Possible causes

1. 01

   Misfires

   Spark plugs, ignition coils, injectors or low compression in a cylinder. A flashing light most often means misfires, and unburnt fuel can damage the catalytic converter.

2. 02

   Vacuum leaks

   Cracked hoses, intake gaskets, a crankcase ventilation valve. The ECU sees a lean mixture and the engine idles roughly. We track these down with a leak test of the vacuum system.

3. 03

   Engine sensors

   Oxygen sensors, mass airflow sensor, pressure sensors, crank and cam position sensors. A faulty sensor sends wrong data and the engine runs worse. It’s worth testing a sensor before replacing it — often the wiring or connector is to blame.

4. 04

   Boost and turbocharger

   On turbo cars — underboost or overboost codes: cracked boost hoses, turbo control, vacuum valves. Usually accompanied by loss of power and limp mode.

5. 05

   EGR and emission systems

   EGR valve, catalytic converter, fuel tank vapour system. The code often appears without any noticeable change in driving, but over time it affects how the engine runs.

6. 06

   Mechanical problems inside the engine

   Low compression, carbon build-up, cylinder damage. If this is suspected, the cylinders can be inspected with an endoscope without taking the engine apart.

How we check

1. 01We connect to the engine control unit and read current and stored fault codes
2. 02We analyse live engine data: mixture, sensors, misfires, boost
3. 03If needed, we test the vacuum system or turbo, or inspect the cylinders with an endoscope
4. 04We explain the cause and repair options and agree on the cost before starting work