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The H98 error means the problem is either going to be the magnetron, inverter or control board. This is not a good DIY fix because factory test equipment will be needed to accurately diagnose the problem plus the unit stores lethal voltage even when unplugged.
I recommend you take the unit down and take it to a experienced workshop to have it fixed. The cost would be anywhere between $150 - $250, Bearing in mind that most new units cost around $229 you may want to look for a new unit.
After the warning given if you decide you want to fix this yourself and have a multi-meter and amp meter I would be happy to assist you further :)
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