Ryobi meaningless gauantee

We purchased a Invertor generator from Buildersware house 18 months ago the generator ran 100% for that time then one mornoing after switching the unit on it accelerated to max rpm and kept doing it so we took it back to builders and they told us to take it to Ryobi directly as it was before christmas and that would be the quickest so we did we were told we would have a reply within 5 days, it never happened, so now in January 2023 we get a quote that is half the price of the generator and then argue the fact that the invertor overloaded therfore not covered by the guarantee. What is wrong when suppliers do not honor their guarantees poor service that we have to endure.