If you are using old kit this might help with the fault but use it gently.
http://uk.rs-online.com/web/p/pcb-cl...20626C6F636B26
If the link doesn't work go to UK RsComponents and search PCB scrubbing block. I wouldn't advise getting this sort of thing from any one else. RS sell to anyone these days as they are aware that they stock items that can't be bought from anyone else. Delivery isn't free but is usually next day.
Might work might not so the choice is yours. It is possible to have slight contact tarnishing cause problems like this. This is also probably the product repairers would use if needed.
There are several authorised Canon and other repairers in the UK. For instance I had a sensor clean and check while I went for a long cup of coffee in Stoke On Trent. There is a list of them on Canon's web site. Afraid I would use one of these. There are too many of the type that just register with some web site about. This makes local ones that I might visit to see what they are like rather hard to find.
John
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