Your first call is to the supplier or manufacturer, not the lender. Faults, defects and warranty issues are dealt with under the sale and the warranty that came with the asset. Importantly, your finance payments normally continue while a fault is being resolved — you generally cannot withhold payment because the equipment is not working.
Where the asset was bought new, the manufacturer’s warranty usually applies as it would on any other purchase. Where it was bought used, the cover may be much narrower, so check what you actually have before a problem arises. Maintenance and servicing obligations are also in your finance agreement on many products, particularly leases, and neglecting them can count as a breach.
Some agreements bundle maintenance or servicing in, especially contract hire. Others leave everything to you. Knowing which you have makes a real difference to your running costs.
If the supplier will not put a serious fault right, tell the lender and tell us. On certain agreements the finance provider can have a degree of responsibility alongside the supplier, and separately you may have rights under consumer or commercial sale law depending on how you bought and who you are. That is a legal question rather than a broking one, so take independent advice if the sums are significant.