Panasonic combi mw fault

Got a Panasonic combination microwave NNCT878SBPQ.

Works on oven and grill function but when you put it on microwave it works for approx 2 secs, stops, then the display goes dead. You can turn it back on and use oven or grill as before but microwave always faults. Customer has already changed inverter and magnetron and we have check the selector flap that goes between oven/grill/microwave. All seem ok.

Customer says fault H97 showed when it first faulted and the invertor was burnt before he changed it. New invertor cleared fault code but still is faulting as above.

Any ideas?