Ford Escort **urgent help needed*** spark plugs sooting up

Hi all spike here,the problem is had problem with car took back to garage they replaced coolant temp sensor & cleared all faults after diagnostics check,car seems to be running all ok,but have taken spark plugs out & they are all sooting up what could be the problem if all faults checked on diagnostics & cleared,could spark plug gap be problem or is it what i think is happening is that the mixture is too rich if this is the case how then was the diagnostics all cleared.The car seems to be running all ok after being checked as before it had probs starting,erratic idle,cutting out & hesitation none of these things are happening now.Please help & advise me.Best regards spike.