Technical Description of OBD-II Trouble Code
Malfunction in the Engine Speed Input Circuit
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to all OBD-II equipped vehicles (Nissan, Dodge, VW, Jeep, Mazda, Honda, Chrysler, Subaru, etc.). Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
A stored code P0725 indicates that the powertrain control module (PCM) has detected a circuit malfunction with the engine speed sensor input signal. The engine speed input sensor is also known as the transmission input speed sensor and the input speed sensor. This code can be caused by either a mechanical problem or an electrical problem.
The engine speed input speed sensor is usually located in the transmission case, near the front of the input shaft. It is typically sealed with a rubber O-ring to prevent leaks. When removing the sensor, caution must be taken as the transmission fluid may be hot and harmful. To avoid any spills, a suitable container should be placed under the opening in the transmission.
The engine input speed sensor circuit operates based on the principle that an electromagnetic hall-effect sensor is mounted in a way that allows a toothed reluctor ring to pass very close to its magnetic tip. The reluctor ring is attached to the input shaft of the transmission, so when the shaft spins, the ring spins as well. The raised areas of the teeth complete the engine speed input circuit electromagnetically, while the recessed areas interrupt the circuit. These rapid completions and interruptions create an electronic waveform pattern that represents a specific frequency and voltage level. The PCM recognizes this waveform pattern as the engine input speed.
If the PCM fails to recognize the appropriate engine speed input signal for a certain period of time and under specific circumstances, a P0725 code will be stored, and the malfunction indicator lamp may illuminate. Unacceptable engine speed input signals may include excessive voltage, insufficient voltage, or incorrect voltage compared to the transmission output speed, throttle position, or engine RPM. In some cases, the transmission control module (TCM) or PCM may enter limp-in mode when this code is stored.
Other engine speed input circuit engine codes that are related include:
Code Severity & Symptoms
A stored code P0725 should be considered severe because the conditions that caused it to be stored may result in transmission damage.
Symptoms may include:
Causes
Possible causes for this P0725 code include:
Diagnostic and Repair Procedures
To diagnose a P0725, access to a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable vehicle information source is required.
When diagnosing a code related to the automatic transmission (anything in the P0700s), start by testing the condition and level of the transmission fluid. Check the fluid level and observe its odor. If the fluid smells burnt, appears extremely black, or has a heavy metallic hue, it indicates possible mechanical failure of the transmission. If the fluid level is low, inspect the transmission, lines, and cooler for leaks and repair them as necessary. Fill the transmission according to the manufacturer’s recommendations and ensure that there are no more leaks. If there is a significant transmission fluid leak from the torque converter area, it may indicate catastrophic transmission failure, requiring transmission removal and possible overhaul.
Once the transmission is filled with the correct fluid and there are no visible leaks, visually inspect the wiring and connectors for any signs of damage, corrosion, or overheating.
With the transmission filled to the recommended level with the appropriate fluid and no visibly damaged wiring or connectors, connect the scanner to the vehicle diagnostic connector and retrieve all stored codes and freeze frame data. Write down this information as it may be helpful during the diagnosis. Clear the codes and, if possible, test drive the vehicle.
If the P0725 code is reset, connect the scanner to the vehicle and observe the engine speed input signal (to the PCM) on the data display screen. If possible, test drive the vehicle and note the engine input speed. If the engine input speed varies significantly from the engine RPM, suspect a defective engine input speed sensor or damage/wear to the reluctor ring.
To test the engine input speed sensor, use the DVOM and follow the testing recommendations and specifications found in the vehicle information source. Also, check the sensor’s magnetic tip for excessive metal deposits and inspect the reluctor ring for imperfections.
Additional diagnostic notes:
