Dear William Thank you for contacting FOBO, Please turn on Gauge pressure in the general setting page and check pressure reading of sensors if you are living and driving in high altitudes area. Gauge pressure is the additional pressure in a system relative to atmospheric pressure. If you observe a difference in the tire pressure reading at high altitudes, this is because FOBO measures in absolute pressure minus 14.7psi (equal to gauge pressure at sea-level) and is not affected by the change of atmospheric pressure. Should you have any issue arise, you may revert to us for further assistance with write an email to cx@my-fobo.com, Best Regards Amir Bayati