Toyota HiAce 2005 Problems

It's most likely to be an engine sensor, or a wiring connection associated with one, that's the problem. Unfortunately, intermittent problems are hard to detect, because they rarely occur when the mechanic is there to witness it.

Normally, I would be thinking it’s the turbo, but the fact the performance hasn’t been affected would suggest it’s not. Check the injectors, it could be that they’re worn and in need of replacement.