Hi,

My car is a 2007 Mazda 6 automatic. I'm the 2nd owner and have been driving it since 2013. I've always followed regular maintenance on it (oil change, brake pads, whatever my mechanic told me was needed, really), although candidly I've never changed the ATF as I wasn't aware there was a need and my mechanic had never recommended it. (I was also not previously aware that this generation of Mazda shares the same transmission as Ford or I never would have bought it in the first place). Anyways, the car has served me well all these issues and never really given me any issues until recently. The AT light came on and it's giving a bunch of error codes:

P0751: Shift Solenoid A Performance Or Stuck Off

P0766: Shirt solenoid D Performance Or Stuck Off

P0733: Gear 3 Incorrect Ratio

P0734: Gear 4 Incorrect Ratio

P0744: TCC Circuit Intermittent

When the AT light came on, the car started having trouble shifting. At first it was just weak shifts, but I was able to use the work-around of driving the car in manual mode and it would shift just fine. Then about 50 miles later (the issue happened when I was a bit away from home), the car started refusing to shift past 3rd gear. Also, when putting the car in reverse, it'd make a "thunk" sound and the car would jolt a little before i'd be able to drive it backwards.

When I first took it to a mechanic (specifically transmission shop), they wiped the code and told me to drive around a bit to see if the code would come back. When the code was wiped, the car drove as if there were no issues at all (shifted up to 5th, didn't jolt when putting it into reverse) for about 10 miles. Then the AT light came back on and the same issues were back again. That mechanic told me I needed to replace the transmission which would cost $4-6k. 

I took my car to another transmission shop for a 2nd opinion and that shop told me that it could either be the shift solenoids like the code is giving, or it could be the TCM, or it could be the transmission itself. When I asked them how do I tell which of those it is, they told me I just needed to try each one until the problem is solved, and recommended I start with the shift solenoids (plus replacing the atf, pan, and filter), and quoted me $2k.

I'm quote confused on what to do because both mechanics gave varying recommendations and neither sound like a solid solution to the problem. Is there any way to actually properly diagnose what the root cause of the issue instead of replacing random parts and hoping it works? I saw from Scotty's videos that sometimes replacing the ATF, pan, and filter can fix the issue, or at least prolong the life of the car enough to drive it for a while longer. Based on the error codes and issues my car is currently giving, would that be a recommended place to start? The ATF is currently between a maroon/brown color.

Thanks in advance for the help!