P0738 code definition
The P0738 trouble code detects an error with the TCM engine speed output circuit
What the P0738 code means
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.
What causes the P0738 code?
Possible causes for the P0738 trouble code are:
- Faulty engine output speed sensor
- Faulty transmission output speed sensor
- Insufficient transmission fluid level
- Dirty transmission fluid
- In rare cases, a malfunctioning PCM
- Defective electrical components, such as wires, connectors, and fuses
What are the symptoms of the P0738 code?
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.
How does a mechanic diagnose the P0738 code?
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.
Common mistakes when diagnosing the P0738 code
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.
How serious is the P0738 code?
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.
What repairs can fix the P0738 code?
Repairs for the P0738 trouble code are:
- Replacement of the engine output speed sensor
- Replacement of the transmission output speed sensor
- Replacement or addition of transmission fluid
- Replacement of wires, connectors, and fuses
- In rare cases, replacement of the PCM
Additional comments for consideration regarding the P0738 code
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.
Need help with a P0738 code?
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