I bought a used 2013 600 Rancher. After replacing the heat sensor in the block, it runs up to the red band quickly while idling. Water pump seems to be working. It really doesn't feel or act hot. When I start driving and air moves across the radiator, the temp drops below half on the scale and stays there. The fan is running. Any ideas? Weak fan? The radiator fluid is warm not hot.

[2015 Z Force 800 53/Trail EPS]

At what point on the gauge does the fan come on? And when it's on, does it seem to be running at full RPM? If it is, it should never allow it to get to be overheated. If so, you may have a stuck thermostat.

The temp sensor on the engine is for that gauge but the fan switch...aka thermoswitch is on the radiator.

At what point on the gauge does the fan come on? And when it's on, does it seem to be running at full RPM? If it is, it should never allow it to get to be overheated. If so, you may have a stuck thermostat.

Like NM says. If your rad water is just warm when your motor is at least at 3 bars on the temp gauge then something is either obstructing the coolant movement or the pump is not working effectively. You should not be able to tolerate touching the rad or the coolant in it at operating temperature.

When you take the rad cap off and rev the motor can you see the coolant inside roiling around?
I find it odd that it cools down so quickly when you start to travel but it overheats despite the fan running. Have to wrap my head around that one

PS should try not to get it into overheat state, these CF Motos really don't like to get hot, they wreck things sometimes.