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P0721 = Output Speed Sensor Range/Performance
It can be caused by a faulty sensor or an internal transmission problem. However in this case I would start with a visual inspection around the transmission and the wiring to it first.
There is a test to check it's resistance, you need a multimeter and the test procedure.
Transmission Output Speed Sensor, Checking
Multimeter
Wiring diagram
With the key off and the transmission in park and everything else turned off.
Go to the side of the transmission and look for the output speed sensor wire connection
It is item 1 in the second image. Inspect the connector for damage or corrosion. Repair/replace as required. You can use the multimeter across the two pins on the transmission side of the connector and see if you get a reading of 1 - 1.5 ohms resistance.
If you do the sensor is possibly good. I don't trust ohm readings only, as it could have a shorted coil in the sensor that stops it from working even though it tests "good". I will hook an oscilloscope to it and actually watch the signal it puts out, but many people do not have those to work with.
If no issues found continue to the next step. This is where you will likely have to stop as disconnecting the transmission control module requires the factory scan tool to read the calibration and programming info from it before the power and connector is removed. Without doing this the anti-theft system will trigger and you will suddenly have a car that doesn't even start.
https://www.2carpros.com/articles/how-to-use-a-voltmeter
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Wednesday, January 23rd, 2019 AT 12:25 AM
