Hi,
on my 1,6 Stilo the MIL light came on, 2 days ago.
So I read the faults with FES:
Pre-cat O2 sensor - low signal
So I did livedata graph of pre-cat and post-cat O2 sensors on idle. (att. below).
And you can see some down spikes to the value 0,00V. This is not normal, it schould oscillate between cca 0,15-0,8V. It was ok in past (I have older graphs), it now started.

I'm not sure what does it mean - car still running fine, with full power, no cutting there.
These down spikes indicate too much oxygen in exhaust, it can be caused by misfires (but I dont feel them) or maybe by dirty injectors? Because dirty injector = less fuel = not every oxygen in cylinder is burned??
What do you mean?

Hi Hanzs
Both O2 sensors are functioning properly with flip flop around every second and pre cat is generally oscillating nicely and post cat is generally fairly steady. So nothing is wrong with the O2 sensors and nothing wrong with the mixture

The sudden drop to zero and recovery happening on both o2 sensors is so quick that it is too fast for it to be the exhaust gas changing. Signal is lost and recovers within microseconds so it's not an O2 sample reading, it's much quicker than their normal sample rate so it's not air/fuel mixture, it's an electrical loss of connection, not a connection between the ECU and the O2 sensors but probably between the ECU and earth

Fault code is stating O2 sensor voltage low, that's exactly what it is and that's what you are looking for, what is causing the voltage to suddenly spike low? You'll have to find out.

My first inspection would be on earths, especially from ECU to battery

Put a voltmeter between O2 sensor earth (NB not the black wire- that's O2 sensor signal) and battery earth and see if you can see spikes.They might be too fast for a multimeter though

This is where LTFT and STFT figures could really be useful but for some reason FES doesn't do them even though they are available on OBD.