GE microwave My Microwave doesn't cook any more. The light is on, the turntable goes around, and the fan works.

see this causes and fix it. God bless you
Magnetron If the microwave is loud or noisy only when cooking, the magnetron tube might be defective. The magnetron uses high voltage, high current DC power to generate the microwave frequency that cooks the food. Under normal circumstances the magnetron is nearly silent. However if it is damaged or nearly burned out it can begin emitting high pitched or growling noises. It's not normally dangerous to use a microwave oven when it makes these noises, but they are an indication that the magnetron is almost dead and will need to be replaced.
Exhaust Fan Motor If the microwave is loud when running the fan motor might be
Stirrer Motor If the microwave is loud or noisy the stirrer motor might be worn out. The stirrer is a metal blade which slowly turns around, deflecting the microwave energy in a random pattern inside the microwave oven. As the motor nears the end of its useful life it begins to make grinding noises. The microwave will not heat food evenly without this stirrer, the motor should be replaced.
Turntable Motor If the microwave is loud or noisy the turntable motor might be worn out. This is a common problem and relatively easy to replace.

first suspect is the inverter board (power) and magnatron a distant second----in either case the part involved unless an expensive unit is pricey.

The problem is more than likely in the microprocessor control board, sometimes a bad relay, or poor solder points.

The door switch and safety switch is not adjusted correctly or is not operating at all.

It's probably your bottom door switch. See this helpful file.

You can download GE owner's manuals and installation instructions here.
You can find links to helpful exploded view diagrams and part ordering help here.

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you have a bad door switch, probably the bottom one

You are correct the fan motor is either stuck or needs replaced.

There is no way to do this without removing the entire microwave. In case your not familiar with how microwaves work, there is a capacitor that is very large inside and can hold a charge even after it is unplugged for up to 24 hours, please take action to prevent getting shocked.

if everything works except the heating, it is most likely the mag tube is on its way out.........that is the most common reason for it not too cook.......since the microwave is a GE and it is 3.5 years old, i would say you are better off buying a new microwave, the mag tube starts at around $70.00 and up + the labor it costs too have a service person put it in........around another $60.00

More than likely the magnetron has failed. This could be for a number of reason. Do you use your microwave to heat wheat packs or warm plates? If so, you may have dried the magnetron out (they use the moisture in whatever you're cooking to heat it, and plates/wheat bags have no moisture).

I'd say a trip to your local Microwave repairer could be in order. It could also be the High Voltage Fuse which has failed. Either way a repairer will be able to tell you!