I’m here asking for a friend. His description of the problem and his description of the mechanics diagnosis don’t add up to me.

2010 Mazda CX-9. 6 speed. Awd. 6cyl. 151,000 miles.

When driving on the highway in “D”, if you slow down the vehicle will chug. It’s described as the same feeling you get in a manual trans car if you don’t downshift when you should.

At first it sounded to me like the auto trans wasn’t downshifting. But....

If the vehicle is driven in manual mode, and if you’re on the highway and slow down it does not chug. Even if you leave it in 6th gear. To be clear - it does not chug even if you leave it in the highest gear.

Mechanic said he thinks it’s either something internally wrong with the trans that is severe enough to merit replacing the trans. Or. He says it could be something to do with one of the multiple CV joints but, he claims, it would be difficult to pin point which CV it is and therefore he’s afraid he’ll be hunting around replacing parts and running up the bill without solving the issue. Either way, the mechanic claims, it’s probably not worth investing the $$ into a car with this many miles.

I’ve driven a car (FWD) with a bad CV. To me, this does not sound like that. I’ve also driven a car that won’t downshift. In my limited experience it’s usually a bad sensor or vacuum leak or lose wire. The fact that this happens in “D” but not in manual mode has me stumped.

One idea I just had typing this up. Is it possible the converter won’t lock up in manual mode? Maybe the chugging is the converter not unlocking? Still doesn’t seem like the right answer to me.

Thoughts?