Hi, had trouble lately with losing coolant, car only overheated for the first time yesterday when temp gauge went past 90' and engine lights came on saying stop check coolant, I got the AA breakdown out today to check car he done a presure test but found no leaks, he checked coolant as it was a brown colour and said he thinks it's the head gasket,but after more tests like letting engine run for a good half hour says everything seemed fine, the heaters were heating up inside ok, he also done a test to see if there was any exhaust gases in the coolant but all was fine, any ideas