Mitsubishi ASX Gearbox & Transmission Problems

It sounds like there’s something pretty major wrong inside the transmission. The lack of drive in reverse is allowing the engine to 'flare' (rev up) because there’s no load on the engine when this happens.

But before you do anything else, check the level of transmission fluid in the gearbox to make sure it’s not low. A low fluid level can cause these symptoms, too, so you need to rule out the easy stuff first.

The ASX was launched in Australia in 2010 and from the very start was offered with a CVT transmission, as well as a conventional manual transmission, depending on what specification you chose. The ASX’s CVT differs from some by having six fixed steps which makes the car feel and sound a little less alien to drive, but also potentially sacrifices a small degree of efficiency. That’s because the CVT is most efficient when allowed to use an infinite range of just-so ratios, rather than the six fixed ones Mitsubishi specified.

Intermittent problems like this one can be very annoying as they rarely emerge when a mechanic is looking at them. However, you need to have the Mitsubishi dealership mechanic with you for a test drive as you try to make the problem occur, however long that takes. Perhaps you could video the car's behaviour next time it happens and show the workshop that way.

Common causes for a manual car to refuse to select a gear are usually down to a worn, maladjusted or damaged clutch or a poorly adjusted gear linkage. If there's a horrible graunching noise when this happens, I'd suspect the clutch, if not, it could easily be a simple matter of adjusting the selectors. Either way, this shouldn't amount to rocket science in 2024.

Just be sure not to take no for an answer; if the car has a problem, it's under warranty and it's Mitsubishi's problem, not yours. 

It’s likely to be the CVT transmission, that’s obviously what the dealer thought. You could pursue it further with dealer/Mitsubishi, but I reckon you’re stuck with it.