Optical isolator - as it says on the tin.. Can you reconfigure the 232 port on C4 to do something else (doesn't matter what) to prove it works properly? Might be putting out ***** and killing the panel?
Any recent C4 firmware updates been applied? Try updating panel firmware? Do you have a ground between C4 and panel on 232? Does it still do it when you physically disconnect C4 and set from keypad?
We have 2 or 3 electricians who we work with and they will always put the spurs in for us and test. I actively use the fact we do not install the spurs (as we're not qualified and/or more importantly competent ) and those other firms just chucking in spurs are probably breaking the law.
Those non-electricians who install spurs - do you test the work you've done and do the paperwork etc? Thought not.
see, yet again assumptions. you're assuming we have alarms in london aren't you. we do other things than just alarms and they don't require 4 hours onsite.