type on your search engine Polaris sportsman 450 recall.There should be something about atvriders.com They have this info on the recalls on the ecm.All the vin numbers and models.It is for the 2006 450 Sportsman among others
I have a 2005 Sportsman 400 and had the same problem. Disconnect the temp sensor and connect a 500 to 1K potentiometer to the cables that were connected to the sensor. If you turn the pot so that the resistance falls below 300k, the fan should turn on. If you turn it back up, it will turn off. If it does not work like that, the ECM is bad. I had the same problem and have two identical units. The pot troubleshooting method works perfectly. Luckily, I had another unit in order to borrow the ECM and verify 100%.
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