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2009 Mazda 3 Hatchback downshifts from 5th to 3rd on interstate when in heavy rain

As the topic states, I will be cruising down the interstate at 60 or more mph in my '09 Mazda 3 hatchback when it will suddenly downshift roughly into 3rd gear and will not leave 3rd until I have turned the key to off and restarted the car. The AT light that lit upon the sudden downshift will turn off and the car will drive normally again. It has the 2.3l engine paired with the 5-speed automatic trans. This rarely happens under normal driving conditions. When it’s raining heavily, it almost always happens. Recently I was driving on interstate in semi-heavy rain and had to pull off the road 3 times in a row to turn off and restart the engine after reaching cruising speed and experiencing the sudden downshift. any ideas?

Have the tires on the vehicle inspected for wear.

If for any reason any of the tires starts losing traction, the traction control system will command the transmission to down-shift to a lower gear to maintain traction.

Tester

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The transmission is going into limp mode, there should be a fault code stored in the transmission control module.

The first thing I would check are the electrical connections to the transmission. It sounds like something is getting wet and messing up the signals. Is the splash panel under the motor still intact?