It could be several things: faulty coolant temperature sensor (as mentioned), tired/lazy fan relay, wiring harness/connector fault, ECU fault (let�s hope not).

A dicky coolant temperature sensor will often throw up an error code 21 and the engine trouble light will illuminate, but not always. The wiring/connectors can be checked visually for any abrasion and places where the fan circuit may be grounding. The fact it�s cycling on and off suggests a switching fault or intermittent short/open circuit. Is the pulsing regular or more random in nature? There is a test for the switching side of things - The relay, fan and to a certain extent its wiring can be checked in the diagnostic mode. It�s simple to do and is probably where I�d start first�

Enter the diagnostic mode in the dash by holding select and reset whilst turning the ignition on and holding the buttons down for 8 seconds (make sure kill switch is set to run when doing this bit). �DIAG� will appear in the display. You can use the select button to switch between �CO� and �DIAG� � you want �DIAG�. With �DIAG� in the display press select and reset together for 2 seconds to enter the diagnostic mode. Then flick the kill switch to the off position. You can now scroll up and down the list of diagnostic codes (checks) with the select (down) and reset (up) buttons. You need to find diagnostic code 51 which is the fan circuit test.

To run the diagnostic test just flick the kill switch to run and the ECU should provide a signal for the fan relay to energise and operate the fan. The test cycles the fan five times on and off. This will determine whether the ECU�s capability to switch the circuit, together with the relay operation is OK and if fan itself is doing what it should.

If all is good, then next have a look at the wiring and move on to the coolant temperature sensor as suggested. You can test simply for an open circuit at the sensor whilst still in the bike, but as I mentioned earlier the engine trouble light/error code 21 would already be indicating this if it was the problem. If the resistance of the thermistor is out of spec then the most accurate and informative method to test the resistance is by removing it and measuring the resistance range whilst it�s suspended in pan of water, heating it up gradually and taking measurements using a thermometer for cross-reference.

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