Engine Idle Speed Selector Sensor/Switch Circuit High
The P256D code indicates that the Engine Control Module (ECM) has detected a high voltage signal from the idle speed selector sensor or switch circuit. This sensor is typically used in diesel engines to allow the driver to select a higher idle speed for auxiliary equipment operation. A high voltage condition suggests a short to power, a faulty sensor, or a wiring issue. This can cause the engine to idle at an incorrect speed, affecting drivability and potentially leading to increased wear or stalling.
01. Most Common Causes
- Faulty idle speed selector sensor/switch
- Short to power in the sensor circuit
- Damaged wiring or connector
- Corroded terminals
- ECM internal fault (rare)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Engine idle speed higher or lower than normal
- Engine may stall or hesitate
- Poor fuel economy
- Inability to adjust idle speed manually
03. Step-by-Step Diagnosis
Step 1: Use an OBD2 scanner to confirm the code and check for additional codes.
Step 2: Visually inspect the idle speed selector sensor/switch and its wiring for damage, corrosion, or loose connections.
Step 3: Test the sensor/switch voltage and resistance with a multimeter; compare to manufacturer specifications.
Step 4: Check for shorts to power or ground in the circuit; repair as needed.
Step 5: If all else is normal, replace the idle speed selector sensor/switch and clear the code.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is generally not recommended to drive with this code as it can cause erratic idle speed, stalling, or poor performance. Have it diagnosed promptly.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150, including parts and labor. The sensor itself is usually inexpensive, but labor costs vary.