[GavinO]

I am posting some more info in case anyone else experiences the problems I have had when the thread of the post-cat lambda sensor stripped during an exhaust replacement on my 2003 S40 GDI. The GDI engines were made by Mitsubishi and therefore share most operational components with Mitsubishi GDI models (e.g. the Carisma). For the rear sensor, the Mitsubishi part number was MR507550, which was replaced by 1588A013. This corresponds to Denso 234000 8101 and 234000 8102. It seems that the 8101 and 8102 variants are interchangeable: I have seen adverts for used parts from both Volvo and Mitsubishi vehicles in the 2001-2003 date range with both part numbers. The current Denso code for this sensor is DOX-0420, so if you can find one of those you should have the correct part. Looking at the Denso website, by analogy (although I have no first-hand experience of this), the front (pre-cat) sensor would be DOX-1359.
I am told that because the parts were supplied by Denso to Volvo under licence Denso don't list them as being suitable for Volvo cars and you won't find them listed anywhere online if you try to search for something suitable for a Volvo.
Unearthing this information was a life-saver: I bought a Mitsubishi one new on ebay for a fraction of the price wanted by Volvo or by numerous suppliers who were trying to sell me the wrong sensor (usually for a non-GDI engine). Lo and behold, it was marked Denso 234000 8101. The GDI engine management system seems to be particularly good at objecting to anything that isn't exactly the right part, but after trying three wrong parts my warning light has finally stayed out!