Often diagnosticians mistake the vehicle being stuck in one gear with what is known as LIMP MODE. The gearbox computer has the capability of sensing a potential problem and it therefore puts the vehicle into a safety mode called limp mode to avoid damaging the gearbox further. DO NOT drive the vehicle unnecessarily in limp mode. You may drive the vehicle a short distance to your workshop but no more than that. Should the vehicle go into limp mode while driving. Pull over, switch off, switch on. The vehicle normally resets to normal mode but will store the fault code on the computer. Normally a vehicle will go into limp mode due to an electrical problem, computer problem, TCM (Transmission Control Module) or even engine problems.
A test drive may be done to record when the vehicle goes into limp mode.