What does the vauxhall signum exhaust emissions indicator mean?

Answer 1

The light comes on when the engine management computer detects a fault with one of the sensors used to run or monitor the engine, or detects that the reading that one of the sensors is giving indicated the engine is not running correctly, and is producing excessive amounts of polluting emissions.

For example:

The system expects a reading of some sort from all sensors at all times, and also expects the reading to be be somewhere between an upper and lower limit.

There will be a sensor monitoring the Crankshaft speed and position, and another sensor monitoring the camshaft position and speed.

Normally the camshaft speed will be half that of the crankshaft. Should the system get a speed reading from of the two sensors that is ZERO, while also getting a reading from the other sensor that is not zero it will know that there is a fault.

As an accurate reading from both sensors is needed to accurately manage fuel injection/ignition timing, minimise emissions and maximise fuel economy the warnign light comes on, as the system is not working as it should.