Bosch 350 Combi. I have a similar fault to an earlier posting in that domestic water is OK but central heating won't operate (pumps run, fan fast mode but no ignition) just pumnps cold water, fan runs even after switching CH off. I suspect CH sensor but have disconnected this, fan stops runnning all other symptoms reamin. Some drips from both auto vent and low pressure sensor. I intend to replace CH sensor, auto vent and low pressure system switch but dont think that this is the end of the problem. Welcome any input, It's getting cold.

Sedrick, December 2006

It was the CH sensor. It had developed a high resistance fault. Tested it with a lower value (fixed) resitor and later the specified thermistor, the boiler funtioned

Sedrick, January 2007