First of all, ask your dealer, or (chevrolet dealer) as autoboxes nowadays need to be done following a set procedure. do not do it on the cheap. if in doubt, call chevrolet 0800 060606 who can direct you to someone competent, and to save yourself further potential problems make sure the cam-belt has been changed. I had a lacetti estate, first chevrolet, on my 4th now.
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