A proper Italian car this - waits until I have 1,000,000 things to do and then decides not to start.

I have a couple of questions somebody may be able to answer.

Most popular fix is the crank sensor - and I have one- but has anyone done one with the engine in situ? It is supposed to be down near the starter motor. Did you go in under or over?

Will my Barnacle objects work with some other (preferably free) software so I can get a reading? It has the small ECU up on the throttle body.

You can borrow my ACR code reader if you like, and like the Coupe I would imagine you access it from below, have you checked for any online manuals, Alfa forums (fora?) Does the injector light come on? And has the engine ever stalled when running? Cam belt changed recently perhaps?

Thanks for the kind offer Dave
Unlike the Fiat it seems to have different indicators. There's a yellow engine shaped one, a trianglish one, and they all go out.
I've been out jiggling wires etc. and it is now starting on the button poised to find a really critical moment to fail.

The engine has never stalled when running, but it did this non start thing a few weeks ago. I thought I'd flooded it and cranked through to a start with the throttle floored.

One thing that might be a clue is that the rad fan comes on when I turn the ignition on.

Is it a 2 litre twin spark engine? Not too familiar myself, do you know what ECU it has? But I assume it has a cam/phase sensor and either that or crank/rpm sensor could be failing intermittently. although' lights are ok now and it starts, when it doesn't start does any warning light come on? (engine one probably, triangle prob to do with ABS?)

Edit: just checked a 156 manual online, triangle with circle/arrow around is VDC vehicle dynamic control whatever that is

It is a 1.8 TS. I think the ECU is a Bosch Motronic 2.10.something?
The lights go out when it fails as well as when it starts.

I think it's Bosch ME7.3.1 but anyway if engine light goes out it could be a fuel fault: leaky injector, flooded engine again?

You're probably right about the ECU rev. It is a CF3 engine anyway.

I can hear the fuel pump come on and go off for 2 secs.
It isn't really a perfect crank sensor problem because they're supposed to fail when warm.

I tried disconnecting the temp sender when it had failed and that didn't fix anything.

Does it fail on cold starts, warm starts, hot starts, or not enough of a pattern to tell yet? On the Coupe, the fans coming one is usually a failed temp sensor itself, or the low coolant temps caused by a failed thermostat (where the ECU thinks the coolant temp is higher and hence puts the fans on). On the 156 does it seem like the thermostat is OK? When the fans come on, does the engine warning light come on? If it then starts, does the fan switch off straight away?

It has been cold starts up to now.
Thermostat is OK but has been showing a little bit cool on the gauge on the motorways lately. Not off the scale, just 10 O'clock rather than 11 O'clock.

I probably need to digest the manual. There doesn't seem to be a warning light that doesn't go out, whether the engine's working or not.

I think the fan kept going when it started this morning.

Update.
It would't start the other day and the man from the AA diagnosed the crank sensor failure before attaching one of those excellent bar things and towing me home.
I've just changed it and the old one was completely open circuit.
Later 156s and 147s have an L shaped sensor with a connector directly on the back.
It lives under the starter motor so is a job you do by feel only.

Thanks for your help Dave.