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Hi. The key to determining what's wrong relies on some patience and the use of a voltage meter, in my opinion. Art Benstein was was kind enough to provide me with the following link for more information on ECUs.
http://www.brickboard.com/RWD/volvo-990665.html
Art also informed me that the culprit in the 561 failures is not a power transistor, but a chip, so my apologies for that bit of misinformation.
As to replacements, white label 561s are generally accepted to have the fix for the problem. You can also substitute 933 units (with EGR), 935 and 951.
Good luck!
DS
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