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P0725 Engine Speed Input Circuit Malfunction

OBD-II Trouble Code Technical Description

Article by
Stephen Darby
Stephen Darby
ASE Certified Technician

Engine Speed Input Circuit Malfunction

What does that mean?

This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to all OBD-II equipped vehicles (Nissan, Dodge, VW, Jeep, Mazda, Honda, Chrysler, Subaru, etc.). Although generic, the specific repair steps may vary depending on make/model.

The presence of a stored code P0725 means that the powertrain control module (PCM) has detected a circuit malfunction with the engine speed sensor input signal. The engine speed input sensor is sometimes called the transmission input speed sensor and the input speed sensor. This type of code may be caused by a mechanical problem or an electrical problem.

The engine speed input speed sensor is usually inserted through the transmission case, near the front of the input shaft. Typically, a rubber O-ring is fitted onto the sensor housing so that the sensor makes a seal with the transmission case. Use caution when removing the sensor from the housing as hot transmission fluid may be harmful. Place a suitable container under the opening in the transmission to catch any fluid that may pour out when the sensor is removed (for testing or replacement).

The basic theory of operation of an engine input speed sensor circuit is that an electromagnetic hall-effect sensor is mounted in such a manner as to allow a toothed reluctor ring to pass in very close proximity to its magnetic tip. The reluctor ring is mechanically affixed to the input shaft of the transmission so that when the shaft spins, it spins in conjunction. The raised areas of the teeth serve to electromagnetically complete the engine speed input circuit while the recessed areas between the teeth interrupt the circuit. These rapid completions/interruptions form an electronic waveform pattern that represents a particular frequency and degree of voltage. The waveform pattern is recognized by the PCM as engine input speed.

If the PCM fails to recognize the appropriate engine speed input signal, for a set period of time and under certain circumstances, a P0725 will be stored and a malfunction indicator lamp may be illuminated.

Unacceptable engine speed input signals may include excessive voltage, insufficient voltage, or incorrect voltage as compared to 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.

Related engine speed input circuit engine codes include:

Code Severity & Symptoms

A stored code P0725 should be considered severe as the conditions which have caused it to be stored may result in damage to the transmission.

Symptoms may include:

Causes

Possible causes for this P0725 code include:

Diagnostic and Repair Procedures

In order to diagnose a P0725, I would need access to a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable vehicle information source.

When diagnosing an automatic transmission related code (anything in the P0700s), I begin by testing the condition and level of the transmission fluid. Remove the dipstick (where applicable) and note the level of the fluid in the transmission. Also note the odor of the transmission fluid. If fluid smells burnt, appears extremely black, or has a heavy metallic hue, suspect that mechanical failure of the transmission has occurred. If the fluid level is more than one quart low, inspect the transmission, lines, and cooler for leaks and repair them as required. Fill the transmission according to manufacturer’s recommendations and make sure that no more leaks are present. If a large transmission fluid leak from the torque converter area is exhibited, suspect catastrophic transmission failure that will require transmission removal and possible overhaul.

Once the transmission is full of the correct fluid and no more leaks are present, visually inspect wiring and connectors for 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 present, connect the scanner to the vehicle diagnostic connector and retrieve all stored codes and freeze frame data. Write this information down because it may be helpful as your diagnosis progresses. Clear the codes and test drive the vehicle if possible.

If the P0725 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 engine input speed. If engine input speed varies greatly from engine RPM, suspect a defective engine input speed sensor or reluctor ring damage/wear.

I would use the DVOM to test the engine input speed sensor by finding the specifications in the vehicle information source and following testing recommendations. I would also check the sensors magnetic tip for excessive metal deposits and the reluctor ring for imperfections.

Additional diagnostic notes:

Related P0725 DTC Discussions

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