Next time you make a warranty claim on a replacement part for your car, be prepared to pay for so-called "diagnostic charges". The garage may try to profit from the warranty work by supposedly spending time trying to work out whether it is actually the part under warranty that failed - even if the warranty covers parts and labour!
In my case, the symptom was a dead speedometer on the dashboard. My friendly local Kia dealer and workshop, Norfolk Motor Group (NMG), quickly worked out that the failed part was the speed sensor attached to the gearbox. They replaced it with an original, new Kia unit and with 12 months warranty (parts & labour) for a total cost of £160. A few months later, the speedometer stopped working again, and I took the car back in - expecting the issue to be resolved as fast as last time, and at no charge to me. Instead, NMG managed to drag the replacement out for a whole month whilst they tried to work out whether it really was the same fault as before, and also charged me £170 of "diagnostic" time. They claimed to have inadequate data on the signals to and from the sensor, so couldn't tell whether it was working or not; they apparently spent a long time emailing Kia support back and forth trying to figure out how it worked - and billed me for this staff training time.
The crazy thing is that I was actually charged more for getting the fault fixed as a warranty claim than for the original fix.
I was stuck with having to pay the bill, or my car would not be returned to me. They had basically decided how much they were going to charge me, and dragged it out with spurious "we don't know how it works so you'll have to pay for us to figure it out" claims. In hindsight I should have put my foot down and told them to get on and fix it - at their cost. If you find yourself being told to pay for more diagnostics time, tell them that it's up to them to isolate the fault and to fix it at no cost if the warranty part has indeed failed. I'd agreed to pay these costs, as long as they were refunded if the warranty part was indeed faulty - but the service manager conveniently forgot that he'd agreed this, so refused to refund me when the car was finally fixed.
Interestingly, NMG does claim to do free diagnostics for warranty work. It just seems to be discretionary whether they decide to stick to that.