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Technical Description of OBD-II Trouble Code
Intermittent Circuit for Engine Speed Input
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to all OBD-II equipped vehicles (Nissan, Ford, GM, Chevrolet, Dodge, Jeep, GMC, VW, Toyota, etc.). Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
If your vehicle has a stored code P0728, it means that the powertrain control module (PCM) has detected an intermittent voltage signal from the engine speed sensor. The engine speed input sensor is sometimes referred to as the transmission input speed sensor or simply the input speed sensor. A mechanical or electrical issue may contribute to the presence of a stored code P0728.
Most engine speed input sensors are inserted through an opening in the transmission case, near the front of the input shaft. The sensor is typically equipped with a rubber O-ring (around the outside) to create a seal with the opening in the transmission case. When removing the sensor from the housing, caution should be exercised as hot transmission fluid may be harmful. It is advisable to place a suitable container under the opening in the transmission to catch any fluid that may leak once the sensor is loosened and eventually removed. This precautionary measure should be taken whenever removing any sensor from the transmission, whether for testing or replacement purposes.
A stationary mounted electromagnetic hall-effect sensor is at the core of the modern engine input speed sensor circuit. The sensor is mounted in such a way that a toothed reluctor ring (attached to the input shaft of the transmission) passes very closely to its magnetic tip as it rotates. The reluctor ring rotates along with the transmission input shaft. The raised areas of the teeth on the reluctor ring complete the engine speed input circuit electromagnetically, while the recessed areas between the teeth interrupt the circuit. These rapid electromagnetic completions and interruptions of the circuit form a waveform pattern. The waveform patterns represent frequency and voltage fluctuations that the PCM recognizes as engine input speed.
A code P0728 will be stored, and a malfunction indicator lamp may illuminate, if the PCM detects an intermittent or erratic signal from the engine speed input sensor for a specific period of time and under certain circumstances. It should be noted that the transmission control module (TCM) or PCM may enter limp-in mode when a P0728 code is stored.
Related engine speed input circuit engine codes include:
Code Severity & Symptoms
A stored code P0728 should be considered severe due to the potential for transmission damage and/or drivability issues.
Symptoms may include:
Causes
Possible causes for this P0728 code include:
Diagnostic and Repair Procedures
To properly diagnose a stored code P0728, you will need access to a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information.
Before diagnosing any automatic transmission related code (anything in the P0700s), it is advisable to check the condition and level of the transmission fluid.
Step 1 – Checking Automatic Transmission Fluid Level and Condition
Refer to your vehicle information source to find the appropriate method for checking the transmission fluid level. While most OBD-II equipped vehicles have automatic transmission dipsticks/tubes, some do not. If the fluid level is not within the specified range, inspect the transmission housing, lines, and cooler for leaks. After repairing any leaks, refill the transmission with the recommended fluid according to the manufacturer’s specifications and recheck for leaks.
Pay close attention to the odor and condition of the transmission fluid. If the fluid has an excessively burnt smell, appears extremely black, or has a heavy metallic hue, it is likely that the transmission has experienced catastrophic mechanical failure. In such cases, it may be necessary to remove the transmission pan and check for excessive debris such as clutch material, metal fragments, and other parts. If the pan contains excessive debris, it will likely be necessary to remove the transmission and perform an overhaul, including the installation of a new torque converter. Once all leaks have been addressed and the transmission is filled with the recommended fluid, visually inspect the wiring and connectors for signs of corrosion, overheating, or other damage. Make any necessary repairs.
Step 2 – Effective Use of the Scanner
If there are no visibly damaged wiring harnesses or connectors, and the transmission is filled to the correct level with the recommended fluid, connect the scanner to the vehicle’s diagnostic port and retrieve all stored codes and freeze frame data. It may be helpful to write down this information for reference during the diagnosis process.
If the P0728 code is reset, reconnect the scanner to the vehicle’s diagnostic port. While test driving the vehicle, carefully monitor the data display screen on the scanner. Narrow the scanner data display to show only relevant data and focus on the engine input speed. If the engine input speed fluctuates inexplicably or varies significantly from the engine RPM, suspect a defective engine input speed sensor or a damaged/worn reluctor ring.
Step 3 – Testing the Engine Speed Input Sensor
After disconnecting the electrical connector from the engine speed input sensor, consult your vehicle information source and follow the recommended testing procedures (using the DVOM) to test the sensor. If the engine input speed sensor does not fall within the specified range, it is defective. If it falls within the specified range, proceed to the next step.
Step 4 – Testing the Engine Speed Input Sensor Signal/Circuit
Reconnect the engine speed input sensor and test the sensor signal by connecting the test leads of the DVOM to the signal wire and the sensor ground wire directly behind the sensor connector. Refer to your vehicle information source and follow the manufacturer’s specifications while carefully observing the sensor signal voltage for any glitches or spikes with the engine running and the transmission in park or neutral. If the sensor signal is within acceptable limits, use the DVOM to test the system circuits between the sensor connector and the PCM connector (see note at the bottom of the page).
Additional diagnostic notes:
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While we always recommend seeking mechanical solutions for this issue, if for some reason that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to function. If you are interested in this option, please upload your ECU file to our portal.
