Hi I removed the heat sink and fan and there wasn’t a speck of dust. I applied thermal compound and re-fitted it. I replaced the backup battery. The laptop started up ok many times. The error had returned this morning though. I turned the laptop onto it’s side and blew into the vent. It then started up ok again ! I wonder if there are certain positions where the fan cannot start ? There are plenty of replacement fans for HP laptops on eBay UK but it isn’t clear if they would be compatible. For example, this looks similar. https://www.ebay.co.uk/itm/HP-17-X-Fan-17-x001ds-17-x012ds-17-x037ds-Cpu-Cooling-Fan-926724-001/163618800125?_trkparms=aid%3D111001%...-d879-11e9-bac6-74dbd1803b4b%7Cparentrq%3A39f4397d16d0aa... I have attached a picture of mine. Thanks