The yellow fuse was gone on the heater so I replaced it and re set the unit,it did the same to start off with but I turned the unit off and on again and it works fine now with a constant 12-7-8 when vans not running,very weird.
I did have the heater running on minimum heat which is not good to do I knew that,I’m wondering if it could have been that after turning it back up to full heat it’s working fine,I’ll turn it on again later from cold and see if it keeps running.