Same code and symptoms on my 2008 +4 (2.0 Duratec) surprisingly Griffo Allon Whites said they have never seen a downstream Lambda failure as it's always the upstream? Logic says the code description would for sure mean the after Cat sensor and that's what had me confused. I noticed that my light comes on every time I am in slow moving traffic for a while or when booting it just after slow moving traffic and never when on a long high speed run so I concur with your analysis that the Cat has simply got cold/cool.
Last week moving around Dorset with 3 others and light constantly coming on, (I leave the lead connected and can clear it at traffic lights now ;-)) 175 miles of main and D/C roads home and no light showing....I shall leave until winter and then change both sensors, belt and braces and all that. ;-)
