sierrafery said:

That's not a risk IMO ... i've never seen a BCU "killed" by an IDM cos then both should have been at fault... if the IDM triggers the crossed B replacing it should fix things and same for the BCU... did you check the data link and it's OK?... also what's the symptom beside the B? it doesnt crank like the OP's in this thread or what?

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Well I hope you're right.

The data link is fine, as are the two obvious black ground cables to BCU and IDM; are there any others? I'm trying to work out which are the power feeds and how to test those.

Symptoms are six bongs on ignition II, crossed B in display, no crank. Also no central locking, no automatic interior lights, no electric windows.

Never mind, it was fuse 13. Either I hadn't checked well enough (probably) or it went while I've been trying to fix.

Oh well, the IDM was corroded, so it's good that I caught that at least.

Naf623 said:

Never mind, it was fuse 13.

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OMG ... that's why you have to check well all the fuses/power feeds before other troubleshooting... anyway, i'm glad it's sorted cos the data link was the last suspect (if everything else was OK which obviously it was not) ... as you realised the hard way one missed blown fuse can lead to complex and hard troubleshooting