There are details and discussion in the XC90 forum Under user shockwave. This includes links the o scope waveforms for the CEM ECU and PEM electronics https://www.matthewsvolvosite.com/forums/viewtopic.php?f=9&t=84648 Pull your part number and edit the title so we can search for it here, too, pl...

The current price of a used PEM on ebay is about $35-$50. Search for a reputable ebay seller. If you have access to junk yard, then it is about $15, which is what I do: getting spare parts from junk yard in case I need them. I have stopped going to jy now bc I think I collect too much junk...lol... ...

The one in the trunk its protected from corrosion and also sits vertically so flakes from under the cooling plate wont fall over the control board. But as above, any used one that looks very clean will work, cheap to test. If you have a junkyard close, the second gen S40 V50 has it under the rear se...

Bad PEM more likely imho.

You need a oscilloscope to troubleshoot the PEM, or risk $25-30 on eBay swap

There is a link to the waveforms in this forum for those with a scope