I just spoke to Jorrit at Auto Forza regarding my conversation I had with the guy who did the emissions test. We agreed that it was a bit peculiar that the CO was high but the air/fuel mixture was OK.
This lead us to wonder what is happening upstream of the catalyst. Jorrit has therefore informed me how to test the Lambda signal; there's a wire loom that feeds to the fuel computer on the right side of the passenger footwell. There's an orange/red harness here with three wires inside... one black, one white and one red. This is a feed for testing the Lamda signal.
Across the white and black wires, measure the voltage with a voltmeter set on a 2 volt scale. Run the engine while monitoring the voltage; a low reading (around 0 volts) is a lean mixture and a high reading (around 1 to 1.2 volts) is a rich mixture. The reading should cycle from high to low about once a second. If it's slower than this then the Lambda probe is getting old and slow and probably needs replacing. If the readings are predominantly low then the engine is running lean. Conversely, if the readings are predominantly high then the engine is running ruch.
I'll carry out this experiment at the weekend; you can read a better explanation of the test here.
If the Lambda probe does look to be getting old, then I just did a quick bit of research. I'm informed that a Maserati OEM Lambda probe is about €250, manufactured my Webber Maranello. It appears, however, that there's a universal sensor, I think manufactured by Bosch, that fits the Maserati Ghibli and is available from Just Lambda for GBP 85 (that's about €130). A bargain!
I'm still interested in installing a wideband probe though... I might even be able to hook it up to my E-Boost 2 to display the Lambda reading... now there's an idea!!!