This OEM sensor made by Bosch for Volvo sits on the end of the S40's fuel rail. On non-turbos, it's all-black with a blue o-ring, & small though it is, can go bad and cause "engine hiccups".
0261230236 or 0261230237 (the Bosch number) will be stamped on your old one, possibly along with 31272733 (the current Volvo part number) or 31251446 (the original one). They're easy enough to install, & there are some replacement tips out there on the web. Note that this part has a high failure rate, so a used one would likely not be very trustworthy.