The light usually indicates that there is some sort of malfunction or a reading from a sensor that is not plausible (e.g. air temperature higher than 70° C).
In a lot of cases its a worn Oxy sensor, there is an inner adjuster that is just able to work within a specifc range, if that range is exceeded, the ECU will store an error code and that light will come alive. During the malfunction, the EC will use a "limp home mode", that means it uses a standard mapping in order not to ruin the engine - that's your loss of power.
On my 33 P4 and on my 75 TS I am able to read out the error codes myself, but don't ask me how it works on a 155.
Maybe your dealer is so kind to read out the error codes of your car without ruining your financial perspective - its a real fast and easy job.
I can promise you nothing but blood, sweat, oil and tears
75 TS 2.0