Diagnosing a cooling system fault on a Zafira 1.8

[peter_wheat]

Hi, I have a cooling system fault light. The cooling system has its own computer controller separate from the ECU. (see: http://www.ebay.ie/itm/1999-2005-Vauxhall-Astra-G-... No cooling system fault codes are being thrown on the main ECU, just the light. I have had the unit out to check the connectors and the internal components for signs of failure, but saw nothing. Also checked the relevant fuses on the wiring diagram which were also OK. The car also has air conditioning which I believe is controlled by the same unit. The radiator has two fans. Have only seen one come on. Not sure if the other fan is to do with the air con? The wiring diagram also shows a 2 wire diagnostic line, which looks to connect to the main ECU. Scanner software shows values for engine coolant temperature. Any diagnostic ideas gratefully received. Many thanks in advance. Pete.