So it might be a stupid mechanic. Let me explain better:

1. check engine light has been on for a few weeks. I checked what it was, it said oxygen sensor.
2. saturday car died, symptoms as I explained.
3. took it to dealership.
4. mechanic told me he checked the ECU, error code for oxygen sensor, and that he restarted the ECU and the car started.
5. he told me the car did not start because of the oxygen sensor, and wanted to charge me the 400€ I mentioned for it.

So likely it was something else, and it started because of luck, or something very simple (a wire somewhere, etc), he corrected it and told me 'if you don't pay me 400€ it's gonna leave you stranded again'?

Mechanics in Spain... not only mechanics, the whole latin attitude (and not doing a generalization) are very 'astute' - many of rip offs, so you gotta be careful.

What do you think? The lambda sensor thing is not going to make ECU stop car from starting? Thus, its something else, right?