Everything works (clock, timer, inside light etc.) but not the fan, turntable and will not heat. It stopped while heating lasagne.

Check if the stepper motor is ok, the fuses (the little ones too) if they are blow first.

whirlpool microwave keep working when door open why

It sounds like the micro generator has died.

Yes, you should buy a new one as the cost of repair is likely to be high.

Most likely you have a magnetron problem. Easy to replace (DYI) but make sure the high voltage capacitor is fully discharged first against the case.

Check to make sure the plastic on the shelve holders or tuntable guide did not wear and expose metal. If these holders or brackets are wore and exposed metal it shorted out your micro wave. They need to be replaced and a fuse needs to be replaced inside the control panel,

The problem is that the magnatron is not getting power. Chance is that if could be a bolwn fuse inside the unit. If the fuse don't fix it then that leaves the charging diode, capacitor or the magnatron, or high voltage xformer of which without technical epertize I would not recomend attempting to replace. good luck

Could be the relay, an internal fuse. With the power off, open up the key panel and search for a fuse. Also, there are thermo sensors black, diameter of a dime and 3/8 inch thick with two wires. Check the fuse and replace if necessary. The thermo can be jumped out to test. Remove the wires, use a paper clip between them and tape it up. Briefly turn the unit on. Replace if it works now. The thermo is generally clamped to the wall of the oven behind the key panel. Never run without the thermo sensor.

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.

The 'Clock, is Machanical just like an ordinary kitchen timer The reast of it is electronic, and at that age time for a new one.

Quite often this is due to a bad door switch. Opening the door while cooking (without first hitting the STOP pad) is the primary cause of door switch failure, but it may also be a broken switch mount. I have a text file called "doorsw.txt" which helps in diagnosing and repairing door switch problems, and a file called "safety.txt" which is of vital importance. It is at our site, which is linked at our listing here on FixYa: http://tinyurl.com/yzjozk You can find helpful exploded view diagrams and order parts by entering your base model number (without the suffix) here: http://tinyurl.com/gv383 or at http://www3.sears.com