Technical Description of OBD-II Trouble Code
The circuit for Transmission Range Sensor “A” is registering a high voltage.
What does that mean?
This diagnostic trouble code (DTC) is a standard powertrain code that applies to vehicles equipped with OBD-II. While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
This is a generic powertrain diagnostic trouble code (DTC) within the transmission sub-group. It is classified as a type “B” code, which means that the powertrain control module (PCM) or the transmission control module (TCM) will not activate the check engine light unless the conditions to trigger the code are present for two consecutive key cycles (turning the key on and off, and then off and on).
The PCM/TCM utilizes the transmission range sensor, also known as the inhibitor switch, to determine the gear position of the shift lever. The code definition itself explains the issue that is causing the code: the signal from the range sensor is either not consistently present or intermittently missing.
Example of an external Transmission Range Sensor (TRS):
TRS image from Dorman
Symptoms & Severity of Code
After the second key cycle, the PCM/TCM will activate the check engine light and force the transmission into a “Limp-home” or “safe-mode” state. This will result in noticeable power loss, especially when accelerating from a complete stop. In this mode, the transmission starts off in third gear, which puts strain on the internal clutches of the transmission.
In my experience, this can cause significant internal damage to the transmission, so it is important to address the issue as soon as possible. It is advisable to avoid using the vehicle until repairs can be made.
Causes
Possible causes for this code to be triggered include:
Diagnostic and Repair Procedures
The transmission range sensor receives a 12-volt signal from the ignition switch and then sends a 12-volt signal, corresponding to the gear position, to the PCM/TCM.
P0709 is triggered when the signal is not detected by the PCM/TCM. The most accurate way to diagnose this DTC is by using a scan tool that can read live data. However, if a scan tool is not available, there are a few things that can be checked using a digital volt ohm meter (DVOM). In my experience, the most common cause of intermittent signal issues is related to wiring.
In this case, it is recommended to visually inspect the wiring harness of the range sensor and thoroughly check the internal contacts of the range sensor connector. If any issues are found during the inspection, the problem should be repaired, the codes cleared, and the vehicle test-driven. If the DTC reappears, set the DVOM to volts and back probe the appropriate circuits at the PCM/TCM while “wiggling” the harness connected to the range sensor. If no intermittent voltage loss is observed on the meter, a faulty range sensor may be suspected.
Related Transmission Range Sensor codes: P0705, P0706, P0709, P0709, and P0709.
