What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to all OBD-II equipped vehicles (Nissan, Ford, GM, Chevrolet, Dodge, Jeep, GMC, VW, Toyota, etc.). Although generic, the specific repair steps may vary by make/model.
If your vehicle has a stored P0728 code, it means the Powertrain Control Module (PCM) has detected an intermittent input voltage signal from the engine speed sensor. The engine speed input sensor may sometimes be referred to as the transmission input speed sensor or simply the input speed sensor. A mechanical or electrical issue can contribute to a stored P0728 code.
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. You must be careful when removing the sensor from the case as hot transmission fluid can be harmful. A suitable container should be placed under the transmission opening to catch any fluid that may leak once the sensor is loosened and eventually removed. You should make this a habit whenever you plan to remove a sensor from the transmission (whether you plan to test or replace it).
A stationary electromagnetic Hall effect sensor is at the heart of the modern engine input speed sensor circuit. The sensor is mounted so that a toothed reluctor ring (attached to the transmission input shaft) passes very close to its magnetic tip as it rotates. The reluctor ring rotates with the transmission input shaft. The raised areas of the teeth on the reluctor ring electromagnetically complete the engine speed input circuit, and the recessed areas between the teeth interrupt the circuit. A waveform pattern is formed by these rapid electromagnetic completions and circuit interruptions. The waveform patterns represent the frequency and voltage fluctuations that the PCM recognizes as engine input speed.
A P0728 code will be stored and a Malfunction Indicator Lamp may be illuminated if the PCM detects an intermittent or erratic signal from the engine speed input sensor for a defined period and under certain circumstances. Keep in mind that the Transmission Control Module (TCM) or PCM may enter limp mode when a P0728 is stored.
Related engine input speed sensor circuit trouble codes include:
P0725 Engine Speed Input Circuit Malfunction
P0726 Engine Speed Input Circuit Range/Performance
P0727 Engine Speed Input Circuit No Signal
Code Severity and Symptoms
A stored P0728 code should be treated as severe due to the level of transmission damage and/or driving issues that may result.
Symptoms may include:
Erratic or inoperative speedometer/odometer
Automatic transmissions may shift harshly (limp mode)
Transmission does not move or moves erratically
Inoperative or incorrect tachometer
Transmission slipping or delayed engagement
Additional transmission input/output speed codes may be stored
Causes
Causes
Possible causes of this P0728 code are:
Open or shorted wiring and/or connectors in the engine speed input circuit
Excessive metal deposits on the sensor’s magnetic tip
Faulty engine speed input sensor or transmission output speed sensor
Damaged or worn engine speed sensor reluctor ring
Mechanical transmission failure causing transmission/clutch slippage
Diagnostic and Repair Procedures
You will need access to a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information to properly diagnose a stored P0728 code.
I like to check the condition and level of the transmission fluid before diagnosing an automatic transmission-related code (anything in the P0700s).
Step 1 – Checking Automatic Transmission Fluid Level and Condition
Using your vehicle information source, find the proper method for checking the transmission fluid level. While most OBD-II equipped vehicles have automatic transmission dipsticks/tubes, others do not. If the fluid level is not to specification, inspect the transmission case, lines, and cooler for possible leaks. Once leaks are repaired, fill the transmission according to manufacturer specifications and recheck to ensure there are no further leaks.
Pay close attention to the smell and condition of the transmission fluid. If the fluid smells excessively burnt, appears extremely black, or has a heavy metallic tint, it is safe to suspect the transmission has suffered a catastrophic mechanical failure. If this is the case, you may want to remove the transmission pan and check for excessive debris such as clutch material, metal, parts, etc. If the pan contains excessive debris, transmission removal and an overhaul (with a new torque converter) will likely be necessary. With no further leaks present and the transmission full of the recommended fluid, you are ready to
visually inspect the wiring and connectors for signs of corrosion, overheating, or other damage. Perform 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. Writing this information down may prove helpful as the diagnosis progresses.
If the P0728 resets, reconnect the scanner to the vehicle’s diagnostic port. Now, carefully monitor the data display screen during a vehicle road test. Narrow the scanner’s data display to include only relevant data and focus on the engine input speed. If it fluctuates inexplicably or varies significantly from engine RPM, suspect a faulty 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 testing recommendations (using the DVOM) to test the sensor. The engine input speed sensor is faulty if it does not meet specifications. If it meets specifications, proceed to the next step.
Step 4 – Testing the Engine Input Speed Sensor Signal/Circuit
Reconnect the engine speed input sensor and test the sensor signal by connecting test leads (from the DVOM) to the sensor’s signal wire and ground wire just behind the sensor connector. Use your vehicle information source and follow manufacturer specifications while carefully looking for dropouts/spikes in the sensor signal voltage with the engine running and the transmission in park or neutral. If the sensor signal is acceptable, 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:
An oscilloscope can also be used to observe live data from the sensor in question
Disconnect electrical connectors from associated controllers before using the DVOM to check system circuit resistance and continuity