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P0738 is a diagnostic trouble code (DTC) for "TCM Engine Speed Output Circuit Low". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $79.99. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.
The P0738 trouble code detects an error with the TCM engine speed output circuit
The P0738 code is a standard OBD-II trouble code that identifies an issue with the signal from the engine speed output sensor in the transmission control module (TCM). Information from the engine speed output sensor is sent to the powertrain control module (PCM), which uses the data to calculate the ideal shift strategy. In general, the engine speed should increase slowly as the vehicle speed is increasing, until the proper shift point has been reached. When the PCM notes that engine speed is either increasing too quickly, or not at all, then the P0738 trouble code will be triggered. The code will also be stored if the PCM is unable to receive a signal from the engine output speed sensor.
Possible causes for the P0738 trouble code are:
The P0738 trouble code is usually accompanied by the Check Engine Light illuminating. The vehicle will likely experience problems with the transmission, such as harsh shifting, an inability to shift at all, and constant stalling. The engine may also misfire, and it’s common for acceleration to be slower than usual. In addition to these symptoms, the speedometer may not work (or may work erratically), and the fuel efficiency will likely be diminished.
The P0738 code will be diagnosed using an OBD-II trouble code scanner. A certified mechanic can use the scanner to view the freeze frame data for the code, and to check for the presence of additional trouble codes. If the mechanic notes that other trouble codes are present, then they should be addressed in the order in which they appear on the scanner. The technician will then reset the trouble codes, restart the vehicle, and check to see if the P0738 code remains detected. If it does not, then it was likely an intermittent error, or possible an inaccurate detection.
If the code remains following the reset and restart, then the mechanic should check the transmission fluid. If the fluid level is too low, then it will need to be replenished, and leaks should be carefully checked for. The failure to note fluid leaks can result in more severe damage down the line. If the transmission fluid is dirty, it can be flushed out and replaced. However, if the mechanic finds the fluid to be excessively contaminated, then the transmission has likely been seriously damaged, and a rebuild or replacement is likely.
Next, the mechanic will visually inspect the electrical components. Any damaged wires, connectors, or blown fuses will require replacement. If any of these components appear to have been damaged by leaks, or other reasons outside of basic wear and tear, then a further inspection will be required. The mechanic should then check the output speed sensors by checking their ground signal and reference voltage.
After any issue is identified and replaced, the mechanic will need to reset the trouble codes and restart the vehicle, to check again for the P0738 code. This helps the technician know as soon as the problem has been resolved.
The most commonly made mistake when diagnosing the P0738 code comes from not following the OBD-II trouble code diagnosis protocol. It is extremely important that technicians follow the protocol at all times, to avoid erroneous repairs. Failure to do so often results in the replacement of the vehicle speed sensor, or even an output speed sensor that is properly functioning.
A vehicle with the P0738 code may not be drivable, or it may be drivable but with severe drivability issues. In rare cases, the vehicle may have no drivability issues at all. However, even if the car drives normally, damage may be done to the transmission if the issue is not resolved. As such, an inspection should be scheduled as soon as the P0738 trouble code is detected.
Repairs for the P0738 trouble code are:
It is very rare for the P0738 code to signal a defective PCM. All other potential causes should be inspected before PCM failure is even considered. In the event that the PCM does require replacement, it will also require reprogramming.
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