Hi Carl,
The airbox definitely would not cause the engine management light to come on. You have some other issue which is being recognised by the ecu.
Essentially an ecu is 'simply' a central brain. Its function is to receive incoming signals, perform a mathematical calculation based on the sensed values and send out the result to some actuators in the field. In the main there are up to 10 input sensors feeding information to the ecu but only 2(diesel) or 4(petrol) outputs to field actuators. In the diesel only the injection timing and the injected amount are adjusted. In the petrol you have 4 injectors (or 1 if single point injection). In some cases they both have one more for the control of the turbo wastegate function.
In all cases whether the signal is incoming or outgoing there is a remote piece of equipment at the far end of the cable. The ecu continually monitors each remote device for electrical continuity and a sensed value within the programmed range. If electrical continuity is lost or the sensor returns a value outwith its range (perhaps because of, say, dirty contacts, resulting in a higher than normal resistance) then the ecu will illuminate the engine management light on the dash and store a code in the ecu which represents the fault. It also usually gopes into 'limp home mode' which, as it suggests, permits just enough engine performance to limp to the garage. A sensor which sees a value within its range whether it be higher or lower than normal will not be recognised as a sensor or actuator failure. It will simply send an alarm to the respective warning light on the dash or if you have one, to the dot matrix display.
Since your car is a series 1 then you will be able to get the engine management light on the dash to flash in a predetermined pattern to signify the current fault. Just look in the self help section for the instructions on how to do this. All you will need is a paper clip.
Once you have determined the fault code please come back and we'll help to fix the problem.
cheers
noz
ps
It would be useful if you put your model and RP number in your signature for future reference. Just click on 'My Controls' at the top right of this screen, 'edit my signature' at the bottom left of the next and fill in the details in the white box in the middle of the following screen.