ECU code 21 help

21 is a knock sensor code. It means you have too low of octane fuel for the application or a lean condition. It could also mean that the knock sensor is failing. I would check your plugs and wash for signs of detonation, otherwise if it continues, you may have a bad sensor.

I just had this one myself. Code 21 ECU code 21 coming up. Sled would start fine, idle etc. but it ran super boggy and just would want to die. (it did this after riding fine for an hour) Then after sitting awhile we started it up again and it seemed to run fine for about 1 hour. Then on the way back down it started again and I couldn't move, idled fine but couldn't touch the throttle. Ended up having to tow her out. Dealer mentioned octane also. Took out all the gas (or at least most) just in case and put new gas in. But then we ended up not being able to get it started again. Artctic Cat at Boyz Toyz in Eagle, CO took care of it by the next day. They replaced the Knock sensor and now it runs just fine. I did 20 miles just today, riding pretty hard. Seems to be cured. All under warranty. Didn't cost a nickle. Had 1500 miles on M8 2010.