Technical Description of OBD-II Trouble Code
Range/Performance Issue with Engine Speed Input Circuit
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to all OBD-II equipped vehicles (VW, Nissan, Chevy, GMC Duramax, Audi, etc.). While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
When I come across a stored code P0726, I understand that it indicates a circuit range or performance malfunction in relation to the engine speed sensor input signal. The engine speed input sensor is also known as the transmission input speed sensor or simply the input speed sensor. This code can be caused by either a mechanical problem or an electrical problem.
The engine speed input sensor is usually inserted through the transmission case, near the front of the input shaft. It is typically equipped with a rubber O-ring on the sensor housing to create a seal with the transmission case. It is important to exercise caution when removing the sensor from the housing as hot transmission fluid can be harmful. A suitable container should be placed under the opening in the transmission to catch any fluid that may spill when the sensor is removed. This precautionary measure should be followed whether you plan to test or replace the sensor.
The typical engine input speed sensor circuit revolves around a stationary mounted electromagnetic hall-effect sensor. This sensor is positioned in a way that allows a toothed reluctor ring to pass very closely to its magnetic tip. The reluctor ring is connected to the input shaft of the transmission. As the shaft spins, the reluctor ring also spins. The raised areas of the teeth on the reluctor ring are utilized to electromagnetically complete the engine speed input circuit, while the recessed areas between the teeth interrupt the circuit. These rapid electromagnetic completions and interruptions of the circuit create a waveform pattern that represents frequency and voltage. The PCM recognizes this waveform pattern as the engine input speed.
A P0726 code will be stored, and a malfunction indicator lamp may illuminate, if the PCM does not receive the expected engine speed input signal voltage for a specific period of time and under certain circumstances.
Unacceptable engine speed input signals may include excessive voltage or insufficient voltage compared to transmission output speed, throttle position, or engine RPM. 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 P0726 should be considered severe as the conditions that caused it to be stored may lead to transmission damage.
Symptoms may include:
Causes
Possible causes for this P0726 code include:
Diagnostic and Repair Procedures
To diagnose a stored code P0726 correctly, access to a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable vehicle information source is required.
Prior to diagnosing an automatic transmission related code (anything in the P0700s), it is important to check the condition and level of the transmission fluid.
Checking the automatic transmission fluid level:
Most vehicles are equipped with automatic transmission dipsticks/tubes, but others are not. Consult your vehicle information source to find the proper method for checking the fluid level. If the fluid level is more than one quart low, inspect the transmission housing, lines, and cooler for leaks. Repair any leaks as necessary. Refill the transmission according to the manufacturer’s recommendations and ensure that no further leaks are present. Take note of the odor and condition of the transmission fluid. If the fluid smells burnt, appears extremely black, or has a heavy metallic hue, it is suspected that mechanical failure of the transmission has occurred. Once the transmission is filled with the recommended fluid and no visibly damaged wiring or connectors are found, visually inspect the wiring and connectors for any signs of damage.
Once the transmission is filled to the recommended level (with the appropriate fluid) and no visibly damaged wiring or connectors are present, connect the scanner to the vehicle diagnostic port. Retrieve all stored codes and freeze frame data, and record this information. It may prove helpful during the diagnosis process. Clear the codes and, if possible, test drive the vehicle. If the P0726 code is reset, proceed to the next step.
Connect the scanner to the vehicle and monitor the engine speed input signal (to the PCM) using the data display screen. Carefully observe the data display screen while test driving the vehicle. Take note of the engine input speed and see if it varies significantly from the engine RPM. If it does, suspect a defective engine input speed sensor or damage/wear to the reluctor ring.
Consult your vehicle information source and follow the testing recommendations (using the DVOM) to test the engine input speed sensor. If the engine input speed sensor does not fall within specifications, replace it. If it is within specifications, proceed to the next step.
Test the engine input speed sensor signal by connecting the test leads (of the DVOM) to the signal wire and the sensor ground at the sensor connector. Follow the manufacturer’s specifications and look for glitches/spikes in the sensor signal voltage. If the sensor signal appears to be in order, use the DVOM to test the system circuits between the sensor connector and the PCM (see note below).
Additional diagnostic notes:
