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Fan switching on with cold engine cb900 hornet


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Hello all,

First time I have come across this problem so wonder if anyone can help. I have a Honda CB900 F2 Hornet. This past week, the fan has been kicking in pretty much straight away and will come on every minute or so during a ride. Coolant levels are good, all hoses and connectors are good. Temperature gauge is showing low (as usual for my journey). When the temperature needle reaches half way, the fan will come on and stay on till the temperature drops as it usually does.

The only problem is that it comes on before the engine is hot. Is it something to worry about do you reckon?

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Perhaps a sensor has had it, or the fan switch is broken in the "on" position.

It's either going to be a engine coolant temperature sensor going doo lally and telling it to switch on, or the fan switch itself is broken and it's stuck on.

Defer to your Haynes manual or an electrical layout of the bike, then find what monitors the conditions around the fan area. Should only be one of a couple of things hopefully, and usually cheap if it's just a small piece of electronics.

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Sounds like the temp sensor for the fan is playing up.......dunno if it's just a thermal switch on the Hornet or if the fan is electronically controlled. Is the bike carb or injected? If it's injected, the fan could be controlled by the ECU. How's the battery voltage? Low battery voltage can do strange things on various bikes..... :wink:

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Cheers guys.

My bike is fuel injected. Cooler weather and a cold garage, I may look first at the battery then and run the optimate over it for a while.

The fan definitely comes on and off, so would rule out the sticky switch. If the battery charging fails to correct it I'll look into the sensor then.

Are these cheap to replace?

Again, thanks,


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