Bought a 2011 mazda cx7 2.5l sport less than 2 months ago. Within the past 3 weeks of driving, it wants to shift really hard into gears, and revs up high before shifting. Wont go into reverse. I changed the transmission fluid, and it shifted and ran fine for about a week, then went back to the same thing and wont go into reverse and is shifting hard in all other gears or revving up before shifting. Any ideas what it could be? Only code that pops up on obd is p0126, which is thermostat.

 

By discription only not the DTC the first guess is you have a transmission that was not serviced and likely has a substantial build up of varnish in the valve body and solenoids as well other parts .

If over 150k miles it’s not like a band-aide single fluid change will cure it.

You also did not state if you did a pant and fluid change or had a shop strive it on a professional transmission service machine.


As for your DTC I suggest going back into the ECU/PCM and retrieve more information you missed. Make sure it is not a blue tooth scanning software but a semi quality Manually connected directly to the OBDII Diagnostic Connector.

 

It only has 128k miles, and i had a mechnic drain the fluid fully and put new fluid in it. Ive looked up a few different things and it mentions the soleniods alot, so im going to try to change them and see if that works. I bought it from a small dealer, and honestly, i dont think it has ever been serviced.

 

You can't drain the complete fluid in that transmission without a special Transmission service machine. This service in most service shops having one takes 1-to-1.5-hour minim labor plus approx. 7-9 quarts of fluid. Draining and changing the filter only removes what is in the PAN. Not such other large oil capacity parts like the torque converter.
Solenoid, Check valves, servos and other internal parts that control oil flow are extremely affected by varnish build up from lack of a normal routine of fluid and filter service. Mostly starting at min. 30k mile and thereafter for the life of an automatic transmission. Naturally this is debatable and mostly among those that never had to deal with costly transmission issues.

If it is a problem with a fluid flow-controlled part like a servo(s) you would need to drop the pan remove the part and bench test it. Some parts are electrically control and can if experienced and have the diagnostic equipment or a high-quality digital volt/ohm meter test them while in the transmission.

And all DTC(s) are a starting point only to diagnosing a problem not the always the actual cause of a problem. A misconception from the internet crowd of DIY and mechanics not really trained in OBDII and CANbus diagnosing procedures.