The VFD does not have so many things to check as you may think. Here is a link to basic testing, most of all test the rectifier. Is it worth it, who knows. A Siemens drive that had a blown part and cost me 20euros to repair was worth it, as it costs new 700euro. If problem was the rectifier it alone costs 200-300. At the same time a chinese drive costs 100 euro. At the other hand the Siemens had maximum frequency of 1000hz i believe and a cheap chinese drive has 400hz max frequency, that also may matter.