Engine: T5: P2178

[Yorky Steve]

My 2004 2.5t V70 I bought in February. It's had a few minor issues which I've got around to sorting, except one. I've narrowed down the circumstances when the above code comes up after CEL illuminates. P2178 is too rich off idle bank..I'm now living in Sacramento CA so the daily average temps this time of year are in the high 90sF. Often triple figures (brutally hot). From cold the car is fine. Even when the ambiant temps are this high. However, each and everytime the car is up to operating temp (and no, it doesn't overheat) and when shut down to say drop off an errand, on restart and pull away the CEL comes on. The data shows around 2000rpm when this comes on. The engine runs/accelerates fine, I changed the MAF a few weeks ago (this was speculative to problem solving, old one was fine, no build up of any significance), the vac hoses are all new in the engine bay from turbo housing to intake manifold. The O2 sensors are within limits. I don't want to chuck money at it obviously, but read somewhere to look at/replace the fuel sensor on the injector rail, but how would that explain it having no obvious running issues? Thanks in advance.