I think this points to an air lock in or near the heater matrix. I have followed the JPEG instructions in the relevant " how to" thread to the letter ( and amazed at how quiet the fans are when they kick in ). I have seen some posts where "rev to 3k for 5 mins" etc etc are quoted. The vents emit hot air after 10 miles or so, but still go cold on turning/ cornering. Can anyone offer any Civic specific tips please ( this is a diesel). Thanks James.