Check Engine light on: causes and what to do
By Altay Hasanli, auto electrician
Short answer
The Check Engine light means the engine control unit (ECU) has logged a fault. The cause can be simple (a loose fuel cap) or serious (misfires, the catalytic converter). If it's on steadily, come in for diagnostics in the next few days; if it's flashing, go easy on the engine and get it checked as soon as possible.
The most common causes
- 1
A faulty sensor
Oxygen (lambda) sensors, the mass airflow (MAF) sensor and temperature or pressure sensors are among the most common causes. The car may drive normally while fuel consumption goes up.
Fix: Computer diagnostics
- 2
Ignition misfires
A faulty spark plug, coil or lead stops a cylinder from firing; the engine shakes and the light may flash. Left alone, this can ruin the catalytic converter.
- 3
Fuel system and injectors
Dirty injectors, a weak fuel pump or a vacuum leak upset the mixture and log lean or rich fault codes.
Fix: Injector cleaning
- 4
A wiring or control unit fault
A broken wire, a corroded connector or a software fault in the control unit can also trigger Check Engine. Even when the code names a sensor, the cause is often the wiring.
What you can check yourself
- Make sure the fuel cap is fully closed: on some cars a loose cap sets a fault too.
- Note whether the light is steady or flashing: that tells you how urgent it is.
- Note any shaking, loss of power, unusual smell or smoke.
- Remember your last fill-up or repair: it helps the diagnosis.
When to stop driving
- If Check Engine is flashing, the engine is misfiring: don't accelerate hard and drive as little as possible.
- If the temperature or oil pressure light comes on with it, pull over and switch the engine off immediately.
- Clearing the code doesn't fix anything: if the cause remains, the light will come back.
How we find the cause
- 1We read fault codes from every module, plus the freeze-frame data from the moment they were set.
- 2We check live sensor readings.
- 3We test the wiring and connectors in the circuit the code points to.
- 4If needed, we analyse ignition, sensor and injector signals with an oscilloscope.
This guide is general information: one symptom can have several causes, and only a diagnostic shows the exact one.
Services and prices
Computer diagnostics
from 20 AZNCar diagnostics: fault codes and sensors
Oscilloscope diagnostics
from 40 AZNAnalysing sensor, ignition and injector signals
Wiring and electrical repair
from 10 AZNShort circuits, sensors and control units
Injector cleaning
from 40 AZNCleaning and bench testing of MPI and GDI injectors
Frequently asked questions
Can I drive with the Check Engine light on?
If it's steady and the car drives normally, you can drive carefully to a diagnostic. If it's flashing or the engine shakes, drive as little as possible.
Can I just clear the code myself?
You can, but the cause stays and the light will come back. Clearing it also erases data that matters for the diagnosis.
How long does a diagnostic take?
A computer diagnostic usually takes 15–30 minutes and starts at 20 AZN. We explain the results and give you a report.
Other problems
- Car won't start
Starter won't crank, clicks, or the engine won't catch
- Battery keeps dying
The battery goes flat overnight or after a few days parked
- Alternator not charging
Battery light on while driving, lights dimming
- Engine shaking or losing power
Shaking at idle, hesitation and loss of power when accelerating
- Electrical faults
Lights, windows or dashboard not working, fuses blowing
- Fuel consumption is up
The tank empties faster than usual with the same driving
- Engine stalls by itself
The engine cuts out while driving, at a stop or right after starting