Microwave not heating unless exhaust fan is on high

A significant failure in a 7-year old microwave usually means replace rather than repair.

Check to see if there's a relay providing power to the fan. If there is, the contacts might be sticking in the closed position. Assuming that this is an over-the-range unit, disconnecting the vent fan completely might not be a good idea as heat/steam from foods cooking on the range would more likely damage the microwave.

There are three common issues, the magnetron "heats food" a cracked high voltage diode, a shorted high voltage capacitor.

Hi,
Here is a tip that will help you to figure out what is wrong with yourMicrowave Oven....

Basic Microwave Oven TroubleshootingTips

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If it is a loud buzzing noise it is either the diode or magnetron. Working with the high voltage of a Microwave oven I would never recommend anyone other than a trained professional to attempt this repair.

If it operates as normal but items don't seem to get hot it probably is a Magnatron failure (microwave producer). More times than not the repair / replacement of a Magnatron these days is not worth the investment, especially if the microwave is 4 years old or older ! You should be able to replace your microwave with a similar featured microwave / hood for about $300 to $400. You might try a JVM1750DMWW by GE great micro/hood for about $300, includes one touch sensor cook capabilities,300 CFM vent fan,removable rack,option to turn the turntable on or off, 1000 watts of cooking power. Good luck !

the fan may be clean, may be fan bush struck thats why fan running not properly, check the internally inside the microwave oven , you should carefull inside micro wave oven very high voltage .