Our Thermador SEC302BB/01 double oven shows F64 error code. This is one that has the hall effect sensor. I read the suggestion to monitor the RPMs of the fan to see if it goes out of spec. I started the *upper oven* cold and watched the display, which stayed in the 2600 - 2800 range for quite a while -- until it got to ~250 degrees -- and then the speed just started going up. In maybe 30 seconds it went up to 4000 RPM and then gave the F64. Any ideas? Seems weird. I don't see a lot of dust or grime on the fan(s).

Also, maybe someone could clear up a couple things that have me puzzled:

• Why does dust make the fan go faster? Seems counter intuitive.
  
• Why do both upper and lower fans run regardless of which oven is running, and does the f64 really refer to just the lower fan?
  

Thanks.