Hello, I hope I can offer some assitance.
Does the car lack power, run badly or is it running OK?
as the engine management light is on the engine management has detected a fault that has occurred at least twice within a preset limit. The only way to pinpoint the cause is to have the fault codes read from the engine management memory using a suitable scan tool. All garages are equipped with a scan tool so ring round to obtain a few quotes, the labour should only be around half an hour at most.
Common causes on the Ka for the light coming on are spark plug leads, ignition coil failures and oxygen sensor failures. As the car is running OK I think you can discount the plug leads and coil as either of these would give the car a misfire and so lack power. Early Ka with this engine fitted (2002 to 2003) had problems with water getting in the wiring but yours should be past this time, a sign of this was the cooling fan running all the time.
If an oxygen sensor has failed when a new part is installed I would have the engine management checked to make sure that the software installed in it is the latest available as the oxygen sensors were revised and if run with early software can cause the light to come back on due to different parameters in the different software levels.
As I said at the beginning get the fault code(s) from the engine management and act on these.
Let me know if I can be of further assistance or if you have further questions.