How do I fix wildly inaccurate compass on iPhone 13 mini under warranty?

nick__t wrote:

It's not just while mounted on the bike and no, there is nothing on the bike. i used it one several. Same problem on each.

Then you may need to contact the retailer who sold the phone to you – or take the phone to Apple or an Apple Authorized Service Provider for evaluation.

Sounds like the only warranty would be through the retailer, though.

Contact Apple's Genius Bar. They can guide you to the next step.

Genius Bar Reservation and Apple Support Options - Apple

You say that the compass goes wild while you have the phone mounted on a bicycle.

By any chance, is your bike set up with a small electrical generator, to convert some of the work that you put into pedaling into electrical energy to run a headlight or a rear safety light? Just thinking out loud, but I believe that a generator is similar to an electric motor, in reverse, and is probably going to generate a localized magnetic field.

(Your car has a generator, too – the alternator – but it's probably far enough away from your phone, and behind enough metal, for it not to interfere with your phone's compass very much.)

Also, how is the phone mounted on the bike? If it's nearly vertical (pointed upwards), maybe that's keeping the phone from sensing a compass direction.

Use the compass on iPhone - Apple Support

nick__t wrote:

How do I fix wildly inaccurate compass on iPhone 13 mini under warranty?

…

Have you got any advice? Do I need to go to a store? Original retailer for repair? The phone was bought from GiffGaff as refurbished but 'like new'. Battery was 100% etc.

It doesn't sound like that iPhone would still be under an Apple warranty.

If there is a warranty, it would be whatever the retailer (GiffGaff) was providing on this refurbished phone.