both the 1.4 and 1.6 engines can suffer from heavy oil use , even the handbook says a litre gone every(###) ###-####iles is acceptable
the 1.8 engines are better but still not to the same level as a VW of this era
ensure that they are running on the correct grade of oil as if the extra thin 0W30 is being used then this tends to make the oil consumption worse
yes the root issue is a poor fit of the piston and insufficient ring tension
the piston was revised in the 1800 to try and help the issue
if the oil use is about the 1000 mile per litre mark then the cat will survive at least 100K miles but you're right it does foreshorten the life
On any car if you get a check engine light or other fault light this typically means one or more of the sensors has failed or has a faulty connection.
The fault can be something quite simple and nothing that’s going to stop the car to something quite major that will reduce engine power and possibly stop the engine
The ECU senses this and may put the car into 'limp home' mode which typically limits the revs and the amount of power available. Usually the car is safe to drive for a short distance as the mode is intended is intended to get you home without incurring any engine damage
The quickest route to repair is to have the car plugged into a diagnostic machine which will read off the fault code stored in the ECU (providing the light is still on at this point) and indicate which sensor has failed or whether the fault is more serious.
You can do a very rough and ready check yourself by unplugging sensors one at a time and seeing if any of them change the driving condition which would indicate the most recently unplugged sensor is the faulty one. This method is not foolproof though and reading a fault code off is the better method.