Mass Air Flow Sensor (MAF)
The fuel/air levels are constantly tweaked by the ECU based on the input from the MAF sensor. If for some reason the MAF sensor is not sending the correct readings then the engine will behave erratically as it will be operating with an incorrect fuel / air mix.
MAF sensors are prone to error when they get dirty. As a car ages there can be a build up on the sensor which can degrade its performance and affect the data it sends to the ECU. The sensor can be cleaned (quite often as a DIY job) to improve accuracy but sometimes they also fail and need replacing.
Engine doesn't start up and/or cuts out when running
The engine doesn't start up very easily and will often not start at all. When it is running it can cut out completely without warning.
Engine is misfiring
The engine sounds like it's not running correctly, the engine sounds rough as if it's not running in time.
Engine does not sound or feel normal
The engine doesn't sound normal, it can sound like it's struggling to keep running or out of time. The engine note produced by the engine can also sound very different, it's often a deeper, more bass heavy sound that it is normally.
Check Engine warning light showing on dashboard
The dashboard warning light showing on this page (or one very similar, sometimes a slightly different shape or colour) has appeared on the dashboard and doesn't go off when the car is running.
Engine is running rough
When the engine is running it does sound like it would normally, it can sound like it's struggling to keep running or produce strange noises that sound like it's not running in time with itself.
Fuel consumption seems above normal range
The fuel gauge seems to be going down quicker than usual, the car is using more fuel when driving in the same way as before.