I my gr 20 is only picking up e a d g b and not the last string ,is there any way i can test it to find the fault please thnk you

If you want help from Roland you will have to register:http://www.rolandus.com/backstage/login.php That will get you access to full support and their email address. I think you are having problems with your GK-3 pickup. Make sure your string that you are having problems with is the optimal height etc. The adjustment for the GR-20 is in the manual at this link:http://www.roland.co.uk/assets/media/pdf/GR_20_Q_A.pdf Make sure your pickup is installed correctly and adjusted properly first. The manual is here:http://lib.roland.co.jp/support/en/manuals/res/1811530/GK-3_e11.pdf If you have everything adjusted and only one string is not working then I would guess you have a cable problem and the best thing is to contact Roland for advice. I didn't walk you through the adjustment because every installation for every guitar is different. Hope this helps.