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If the microwave oven works correctly in all features with one big problem that it does not heat, it likely is a bad a magnetron tube.
Do you hear the fan inside the microwave running when you run the microwave to heat? If so, this is another symptom that points to the magnetron tube.
The magnetron assembly is an expensive component. They may cost almost as much as a new microwave oven. So, we don't usually replace them. The one exception would be if the the microwave were still covered by the manufacturer's warranty.
Some of these were covered for 5 years on the part. You can check your owner's manual or call GE with the model number (found on a sticker on the inside wall) to confirm how long the magnetron tube was covered for.
Please let me know if you would like more information.