Error Code P0728 is defined as Engine Speed Input Circuit Intermittent. This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. This includes vehicle models from but not limited to Chrysler, Dodge, Jeep, Mazda, Nissan, Honda, Subaru, Volkswagen, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model, and powertrain configuration.
Error Code P0728 appears when the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has determined an intermittent input voltage signal from the engine speed sensor. Also referred to as the transmission input speed sensor, or input speed sensor, the engine speed input sensor, this component could experience a malfunction caused by either a mechanical or electrical problem.
The engine speed input speed sensor is generally inserted through the transmission case, near the front of the input shaft. Usually, there’s an O-ring fitted to the sensor housing to make sure the sensor makes a seal with the transmission case. Be careful when taking out the sensor from the housing, as hot transmission fluid can be harmful. Put an appropriate container under the opening in the transmission to catch any fluid that may spill out when the sensor is removed (for replacement or testing).
The most basic idea of the operation of the engine input sensor circuit is an electromagnetic hall-effect sensor, which is mounted in such a way that allows a toothed reluctor ring to pass in close proximity to the magnetic tip. The reluctor ring is mechanically affixed to the input of the transmission shaft. When the shaft spin, it spins with it. 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 PCM recognizes this wave pattern as engine input speed.
Error Code P0728 appears when the PCM detects an erratic or intermittent signal from the engine speed input sensor for a set period of time and under certain circumstances. Keep in mind that the transmission control module (TCM) or PCM may enter limp-in mode when a P0728 is stored.
Other related engine speed input circuit engine codes include:
- Error Code P0725
- Error Code P0726
- Error Code P0727
- Defective or erratic speedometer/odometer
- Erratic or failed transmission
- Incorrect or defective or tachometer
- Harsh shifting of automatic transmission
- Delayed engagement or transmission slippage
Additional transmission input/output speed codes may be stored as well
There are multiple causes for this code, including:
- Faulty engine speed input sensor or transmission output speed sensor
- Faulty or worn out engine speed sensor reluctor ring
- Excessive metal deposits on the magnetic tip of the sensor in question
- Open or shorted wiring and connectors in the engine speed input circuit
- Mechanical transmission problem resulting in transmission or clutch slippage
How to Check
As with most codes, checking with a TSB (technical service bulletin) is a good starting point to diagnose this code. Other things you need include the basics, such as a scanner, DVOM (digital volt/ohmmeter), transmission gauge, and vehicle service manual (or equivalent).
When diagnosing this code, or other codes related to the automatic transmission, such as P0700s, start testing the condition and transmission fluid level. Use the dipstick (if applicable) and take note of the transmission fluid level. Also, take note of the odor of the transmission fluid. If it smells burnt, look extremely burnt (black), or with heavy metallic hue, then you can suspect a failed transmission. If the fluid level is more than a quart low, then check the transmission, lines, and looks for signs of leaks. Repair as necessary. Then, fill the transmission according to the manufacturer’s recommendations, make sure there are no leaks. If a large part of the fluid is leaking from the torque converter area, then you can suspect a transmission failure. This means you will need to remove and overhaul the transmission.
If the transmission is full, and there are no signs of leakage, then proceed on inspecting the wires and connectors. Look for signs of damages, overheating, or corrosion.
With the transmission filled with the right amount of fluid, and there are no faults in the connectors or wiring, then connect the scanner to the vehicle diagnostic connector and get all stored data, including the freeze frame data. Take note of this information, clear the codes, and then take the vehicle for a test drive (if possible).
If the error code comes back, reconnect 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, then you can suspect a defective engine input speed sensor or reluctor ring damage/wear.
You can use the DVOM (digital volt-ohmmeter) to test the engine input speed sensor by looking for the specifications from the vehicle information source. Make sure to follow the instructions recommended by the manufacturer. Also, check the sensors magnetic tip for excessive metal deposits and the reluctor ring for imperfections.
How to Fix
Depending on the diagnosis, common repairs for this code include:
- Replacement of the input speed sensor or output speed sensor
- Repair or replacement of faulty, shorted, damaged, or open electrical components within the engine speed circuit
- Replacement of defective engine input speed sensor or transmission output speed sensor
- Replacement of faulty shift solenoid
- Replacement of defective crankshaft or camshaft position sensor
- Replacement of defective electrical components within the crankshaft sensor circuit
- Flush contaminated transmission fluid and refill with clean fluid
- Repair or replacement of defective valve body
- Replacement of the PCM (rare)
For most cases, vehicles with this code are still drivable. Engine and transmission, of course, can suffer, and then there’s the risk of damaging the vehicle further.
If an oscilloscope is available for the diagnosis, then use it to observe live data from the sensor in question.
Prevent further damage by unplugging electrical connectors from all related controllers before using the DVOM to check resistance and continuity of system circuits.