Lights, fan,etc work but turntable does not rotate and oven does not cook.

you have a couple bad switches

If everything is working normally, except for heating, the likely cause is the magnetron.  Microwaves can be dangerous to work on, as the site below points out.  If you have an ohmmeter, it is possible to check the magnetron, but be certain to follow the precautions outlined at the webpage:
http://www.gallawa.com/microtech/mag_test.html

Please check the door switch for proper operation.

The turntable system typically consists of the motor, the coupler, the turntable support with rollers, and the tray.

Here is a video of how a turntable motor should sound when you rotate it by hand.

If yours is not set to be off via the control panel, and it won't rotate or it sounds like it's grinding, etc., you may have a bad motor or plastic coupler.

To replace either, you need to unplug power, then remove the bottom cover of the oven.

You can find links to helpful exploded view diagrams and part ordering help here.

But if you've recently had trouble with the cooktop lights, then the control board may be damaged.

We regularly repair these control units for customers nationwide by mail for $39.95 postpaid in about a week or so with a one-year guarantee.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

Microwave?

Generic advice follows.

Do you open the door while the machine is cooking? Do it a lot? For a long time (months)? Check your door safety interlock switches. The microswitches can not stand that type of abuse. I have personal knowledge on this. I try not to open the door until it stops, but it is hard usually to do that!!!

If you need more help, let me know.

a

The fact that the microwave and convection both do not heat indicated that the oven is in demo mode, check the user manual, otherwise it will possibly be a relay not triggering, may need a pcb replacement, check Demo mode 1st....Phil

If you look at consumer affairs.com, there are many problems like this with GE microwaves. Not a good track record, experiencing problems with mine as well.

It has a bad magnetron. This is the tube that provides the microwaves to heat the food. Not too easy to replace yourself but it can be done. Cost of magnetron is probably less than $50 or $60.

Does the turntable rotate? If everything but this works (except no heat), you probably have a bad door switch. Here's a discussion of what's involved in replacing it: http://www.fixya.com/support/t267796-kenmore_microwave_hood_combination_over

If the turntable does rotate, I'd suspect a more significant failure, such as the transformer, capacitor, or magnetron. If it's any of these, it's time to replace the unit.

check magnetron related circuit and replace magneron, check condencer, diode and etc.,