Engine Idle Speed Selector Sensor/Switch Circuit High
The P256D code indicates that the Engine Control Module (ECM) has detected a high voltage condition in the idle speed selector sensor or switch circuit. This sensor/switch is typically used in diesel engines to allow the driver to select a higher idle speed for auxiliary equipment operation. A high voltage reading suggests a short to power, open circuit, or faulty sensor/switch, which can cause the engine to idle at an incorrect speed or fail to respond to idle speed adjustments.
1. Most Common Causes
- Faulty idle speed selector sensor/switch
- Short to power in the sensor circuit
- Open or damaged wiring/connectors
- Corroded or loose electrical connections
- Faulty ECM (rare)
2. Symptoms You Will Notice
- Engine idle speed too high or too low
- Inability to adjust idle speed using selector
- Check Engine Light illuminated
- Poor fuel economy
- Engine surging or stalling at idle
3. Step-by-Step Diagnosis
Step 1: Use an OBD2 scanner to confirm the code and check for other related 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 resistance and voltage according to manufacturer specifications.
Step 4: Check for shorts to power or ground in the circuit using a multimeter.
Step 5: If all tests pass, suspect a faulty ECM and perform further diagnostics.
4. 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 speeds, stalling, or poor drivability. Have the issue diagnosed and repaired promptly.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150, including parts and labor. The sensor/switch itself is usually inexpensive, but labor costs vary depending on accessibility.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.