P0728 OBD-II Trouble Code: Engine Speed Input Circuit Intermittent

P0728 code definition

When the PCM stores the P0728 code, it means that the signal from the engine speed input circuit is incorrect in some way. Other codes related to the output speed sensor are often stored with this code.

What the P0728 code means

In order to shift through gears correctly, the transmission relies on the TCM and the PCM to monitor the speed of the vehicle. The PCM expects to see the vehicle speed increasing gradually through the shifting procedure; if the speed does not increase, or if the signal from the engine speed input circuit is erratic or nonexistent, the P0728 code will be stored.

What causes the P0728 code?

Most of the time, this code is stored when the engine input speed sensor, or the transmission output speed sensor, is faulty.

Other causes may include:

• Shorted, damaged, or open electrical components within the engine speed circuit
• Faulty shift solenoid
• Defective engine sensors, such as the engine temperature sensor, or other drivability sensors
• Defective crankshaft or camshaft position sensor
• Defective electrical components within the crankshaft sensor circuit
• Restricted transmission fluid flow due to contaminated fluid
• Defective valve body

What are the symptoms of the P0728 code?

When this code is stored, drivers will likely notice problems with the shifting function of the vehicle. Harsh shifting, failing to shift, or hesitation when shifting are all common. Additional symptoms may include:

• Decreased fuel efficiency
• Fluctuating or failing speedometer
• Stalled engine
• Misfiring engine
• Illuminated Check Engine lamp

Only in very rare cases would no symptoms be observed at all.

How does a mechanic diagnose the P0728 code?

There are two initial processes that the mechanic should start with when diagnosing the P0728 code. The first is to visually inspect all the electrical wiring, connectors, harnessed, and other components, and to repair any that are damaged or loose. The next is to check the transmission fluid for contamination. If the fluid smells burnt, it should be flushed, and the system should be filled with clean fluid.

These two steps allow the mechanic to eliminate two common and inexpensive repairs that can fix this issue. After these and every other repair, the code should be cleared from the PCM, and the system should be retested. If the code returns to the PCM, the mechanic should continue checking the voltage readings from each component that could be at fault, manually comparing them to the manufacturer’s specifications until the faulty component is discovered.

Common mistakes when diagnosing the P0728 code

There are many types of mistakes that could be made when diagnosing the P0728 code. Many technicians report that this code is frequently mistaken for an issue with the transmission, the engine, the fuel delivery system, or the driveline. Others report that the vehicle speed sensor is often replaced unnecessarily, before the electrical components or the transmission fluid is checked.

How serious is the P0728 code?

The P0728 code represents a serious drivability issue, and should be repaired as soon as possible. Without the ability to provide a shifting strategy as the vehicle accelerates, the engine is very likely to not perform correctly, and to need more extensive repairs later on.

What repairs can fix the P0728 code?

Repairs for this code include:

• Replace faulty engine input speed sensor or transmission output speed sensor.
• Replace or repair shorted, damaged, or open electrical components within the engine speed circuit.
• Replace faulty shift solenoid.
• Replace defective engine sensors, such as the engine temperature sensor, or other drivability sensors.
• Replace defective crankshaft or camshaft position sensor.
• Replace defective electrical components within the crankshaft sensor circuit.
• Flush contaminated transmission fluid and refill with clean fluid.
• Replace or repair a defective valve body.

If either the crankshaft or camshaft components must be replaced, most manufacturers recommend replacing both at the same time, to ensure simultaneous wear.

Need help with a P0728 code?

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