Just in case anyone gets this fault. I had it as intermittent and recently it went permanent. ECM 6805 boost pressure too high.
After checking LeeT5s post when he had this I also suspected a sticky Compressor Bypass Valve CBV. I have the forge CBV on my turbo which clearly must be getting dirty and oily in the bottom of the turbo and causing it to stick.
Now I could have spent 3hours taking things apart, taking it out and refitting it which is a real PITA as anyone will know who has fitted a CBV on these cars. Instead I chose to identify the line which feeds it and spray a bit of turbo cleaner down the line.
Below is the read out from VIDA. Apologies if my images are sideways. I canít help it!
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Below is the vacuum diagram. You can buy this sticker from Volvo.
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Below shows the lines clearly. I sprayed down the green one after disconnecting it at the right corner of the inlet manifold.
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And finally the product used.
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Went for a test drive and noticed an immediate difference as soon as I pulled away. Better response and much smoother through the rpm range. Deleted the code, driven for 10mins, rescanned and no fault recorded.
Hope this helps anyone who has the same issue. Most google searches for ECM6805 refer to the Diesel models, hardly any results for petrol cars.