My Automatic grand cherokee limited 4.7litres (1999 Model) runs on only one gear without changing on diagnosing the error code P0731 ( gear 1 incorrect ratio) was reported

If you have a scan tool capable of looking at input and output speed sensor data for the transmission that is the first thing I would look at. If you don't than it wouldn't hurt to try a new input speed sensor. They are available from your local NAPA. The speed sensors are located on the drivers side of the transmission. They are a light yellowish white color. it will have a black connector with two wires. The one towards the front is the input speed sensor the one to the rear is the output speed sensor. you will need a 1"socket to remove it.

If replacing the sensor doesn't fix it you will need to bring it in and have it looked at.

The speed sensors are pretty common failures on Chrysler transmissions.

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