Cause
Operation inhibited at high altitude
A function withheld because the vehicle is operating above the altitude it was calibrated for. Air density falls with altitude, and systems that depend on it — fuel-fired heaters, some regeneration strategies, forced induction limits — are inhibited rather than run outside their envelope.
A fuel-fired auxiliary heater is the clearest case: combustion becomes unreliable in thin air, so the unit refuses to start above a threshold and stores a code. It is designed behaviour and not a fault, which is worth telling an owner who has just driven over a mountain pass. Check the barometric reading in the freeze frame — a code stored at high altitude and never repeated at sea level is the inhibit, and no component needs testing.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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