Toyota iQ Gearbox Slipping

Interesting. I have the same iQ model. Are you experiencing any audible indications, like a whine or grinding sound, especially at lower speeds? I had similar shifting issues a while back; did you check the transfer fluid level and its condition for any sign of metal particles? If that transfer case fluid is the source, it can lead to defective gears. Did you notice any oil leakage?

Would you suggest an oil change as a prudent initial measure regarding potential particulate matter in the lubricant? Could you elaborate on the diagnostic steps you took to confirm the fault's origin, or did you decide on repairing the transfer case?

In my case, simply changing the fluid didn't resolve the problem entirely, although it did improve the shifting. I'm afraid it sounds like you might be facing a similar situation with defective gears. Honestly, I recommend you take it to a qualified workshop for a thorough inspection. It's better to catch these things early before they cause more extensive, and expensive, damage.

I've arranged for a comprehensive inspection by a certified technician. Diagnosis confirms wear within the transfer case, needing repair. The estimated cost is about 330€, including parts and labor. Thanks!