What does it mean?
The diagnostic code P0726 is a generic OBD-II powertrain code that applies to all compatible vehicles (VW, Nissan, Chevy, GMC Duramax, Audi, etc.). Although generic, specific repair steps may vary depending on the make and model.
This code means that the Powertrain Control Module (PCM) has detected a malfunction in the engine speed sensor input signal. This sensor is also called the transmission input speed sensor. A mechanical or electrical problem may be the cause of this code.
Engine Input Speed Sensor Operation
The engine input speed sensor is inserted into the transmission case, near the input shaft, and is equipped with an O-ring to ensure sealing. It is important to take precautions when removing it, as hot transmission fluid can be dangerous.
The sensor works with a Hall effect electromagnetic sensor that detects the rotation of a toothed reluctor ring attached to the transmission input shaft. This process generates a signal that the PCM interprets as the engine input speed.
A P0726 code is stored when the PCM does not receive the expected signal voltage for a certain period or under specific conditions.
Code Severity and Symptoms
A P0726 code is considered severe because it can lead to transmission damage.
Symptoms include:
- Transmission not shifting or shifting erratically
- Erratic or non-functional speedometer/odometer
- Automatic transmission shifting harshly (limp mode)
- Non-functional tachometer or displaying incorrect speed
- Transmission slipping or delayed engagement
- Other stored transmission input/output speed codes
Possible Causes
- Excessive metal deposits on the sensor’s magnetic tip
- Faulty engine input speed sensor or output speed sensor
- Damaged or worn input speed sensor reluctor ring
- Damaged wiring or connectors (short circuit or open circuit)
- Transmission mechanical failure causing slipping
Diagnostic and Repair Procedures
1. Transmission Fluid Check
- Check the transmission fluid level and condition
- Look for possible leaks and repair if necessary
- If the fluid smells burnt or is contaminated, suspect a mechanical transmission failure
2. Visual Inspection of Wiring and Connectors
- Look for damaged wires, corroded or bent pins
3. Using a Diagnostic Scanner
- Retrieve all error codes and analyze freeze frame data
- Clear codes and perform a road test
4. Checking Input Speed Sensor Signal
- Observe the sensor’s live signal via the scanner’s data screen
- Compare engine input speed to engine RPM
- If significant discrepancies are detected, suspect a faulty sensor or reluctor ring
5. Testing Input Speed Sensor with a Digital Voltmeter (DVOM)
- Compare values to manufacturer specifications
- Replace sensor if out of specifications
6. Checking Sensor Circuit
- Test connections between sensor and PCM
- Check wire continuity and resistance with a DVOM
Additional Notes
- An oscilloscope is ideal for observing live signals
- Always disconnect electrical connectors before testing resistance or continuity
By following these steps, you can effectively diagnose and repair a P0726 code to restore the vehicle’s proper operation.