Engine Fault indicator

[hello22]

A week after taking the car for service in a local garage, the Engine Fault indicator is lit on the dashboard. (The car is C180 kompressor, Year is 2005, milage 90k).
A mechanic added engine-cleaning liquid to the petrol tank. The indicator light turned off.
4 weeks later, it is turned on again!!
Could it be that the engine is dirty, or a more serious problem?

 

[NiC63]

Why don't your mechanic just stick it on a star diagnostic to find what fault it is.
It can be a numerous of things.

 

[hello22]

Just to add to the original question. The car is mainly used for short distances. This could cause dirt in the carburator. Could this lead to the Engine Fault indicator turn on?
It will be good if l can solve the problem without the need to go to the mechanic again.