Kitchenaid mixer Professional HD 475 watt, unknown model number. Power switch goes straight to a higher speed in the STIR setting. Need to figure out model to find parts.

SOURCE: My Kitchenaid K5SS 300 watt standing mixer launches into the highest speed regardless of the speed ....

I would recommend that you first check the Hall effect sensor which determines the speed.  I had this exact problem (but mine occurred with a 600 professional model) and I actually went out and bought another Hall sensor for about $13.  I replaced the original sensor and made sure that the connection between the sensor and control board was nice and snug. My blender worked as good as new! But I was curious to know if my original sensor was defective so I put it back in mixer and again made sure all of the connections were tight.  Guess what?  My original sensor also made my mixer work!  There was nothing wrong with my original Hall effect sensor except the connection between the control board and sensor was not adequate--unfortunately I didn't check the connections before I decided to buy another one. My guess is that the connection between the control board and the sensor probably loosens or comes undone over time from use and vibrations. 
1.  Download the mixer parts and diagrams from the Kitchenaid website to see what the Hall effect sensor looks like. 2.  You'll need a phillips head screwdriver to open up the mixer. 3.  Make sure that the connection between the control board and sensor is absolutely tight and snug. 4.  Also, make sure that the orientation of the sensor connection between the control board and sensor is correct. it's hard to know which way the sensor should be connected because the control board has three prongs and the sensor can attach facing up or facing down. If, after you've made the connections, the mixer still doesn't work.  Disconnect the sensor and reconnect it with the prongs facing the opposite direction and try it.  I actually found the right orientation through trial and error.  By going through this trial and error, I also had my Eureka moment when I realized that the original sensor probably would also work it were reconnected with correct orientation.   5.  This whole process probably takes less than 10-15 minutes to perform.

SOURCE: I have a 350 watt

replace the stir speed switch

SOURCE: kitchenaid mixer speed

I would recommend that you first check the Hall Effect sensor which determines the mixer’s speed.  I had this exact problem and I actually went out and bought another Hall sensor for about $13.  I replaced the original sensor and made sure that the connection between the sensor and control board was nice and snug. My mixer worked as good as new! But I was curious to know if my original sensor was defective, so I put it back in mixer and again made sure all of the connections were snug and tight. Guess what?  My original sensor also made my mixer work!  There was nothing wrong with the original Hall Effect sensor except that the connection between the control board and sensor was not tight--unfortunately I didn't check the connection before I decided to buy another sensor. My guess is that the connection between the control board and the sensor probably loosens or comes undone over time from use and vibrations. To fix your problem:
1.  Download the mixer parts and diagrams from the Kitchenaid website to see what the Hall Effect sensor looks like. BTW the part number is 9703312.
2.  You'll need a phillips head screwdriver to open up the mixer gear case cover.
3.  Make sure that the connection between the control board and sensor is absolutely tight and snug.
4.  Also, make sure that the orientation of the sensor is correct.  It's difficult to tell which way the sensor should be connected because the control board has three prongs and the sensor can attach facing up or down. After you've made the connection, run the mixer. If it still doesn't work, disconnect the sensor, reconnect it with the prongs facing the opposite direction and try it again.  I actually found the right sensor orientation through a little trial and error.  But by going through this trial and error, I also had my Eureka moment when I realized that the original sensor probably would also work if it were reconnected with correct orientation.  
5.  This whole process probably takes about 10-15 minutes to perform.
6.  Finally, use your common sense and make sure to unplug the mixer before checking, connecting or disconnecting anything.

SOURCE: KitchenAid 2671 onmly run Stir and Second at high speed

Hi. Goto kitchenaid.com and download the parts catalog for your mixer model, these drawings will assist you in the disassembly and reassembly of your mixer.

You'll need to replace the speed control in your mixer. This is a common problem. Be prepared to wait at least a month for your parts to arrive. Unplug your mixer before you open it up.

If you're still under warranty call kitchenaid!

If this solution has assisted you, please rate it. Good luck! ricardok45

SOURCE: My 325 watt, 5 quart artisan stand mixer won't

By not working to mean that the planetary gear wont move? or does it just make a weird noise when it is on a low setting? If the gear is completely stripped the unit shouldn't run on any speed regardless if there is mixture in the bowl or not. Since I can't see the mixer I would say that there might be a slight tear in the gear which would only allow the mixer to get stuck on a slow speed where as on a high speed it is running fast enough to skip that particular tooth. I understand that the mixer is brand new but when it comes to mixers you can strip a gear right out of the box depending on the consistency of the batter you are mixing. If the machine is under a year old it is under warranty and you are allowed free service by an authorized repair center in your area.