We have a preowned 2009 XC70 AWD D5 with an automatic. It has problems that appear to be related. The warning for DSTC service required is on. It automatically shifts into 3rd gear and will not change out, even manually. The engine runs rough as though not all cylinders are firing.

We took it to the local garage and they did some kind of "electrical" repair that clearned the problem for them. 2 miles after picking up the car, all three symptoms returned.

Their suggestion is to replace the wiring harness in the engine compartment. With labor, that could cost 4,000 to 5,000 because they'd have to pull the engine. That sounds to me like they are swinging blindfolded at a piñata. Unless there is a rusted or loose connector, not sure what replacing the whole harness could address.

Any idea what could be going on? Sensors? Rusted connectors? Software?