What Does P256D Mean?
The PCM detects abnormally high voltage on the idle speed selector sensor/switch circuit. The signal voltage exceeds the maximum threshold for any valid switch position.
Common Causes
35%
Open circuit in ground path for switch
30%
Short to voltage/power in signal wire
20%
Faulty idle speed selector switch with open internal circuit
10%
Corroded or disconnected ground terminal
5%
PCM internal circuit failure with excessive pull-up voltage
Diagnostic Steps
1
Step 1: Measure voltage at idle speed selector switch connector with key on (note if voltage is at battery level or reference level)
2
Step 2: Check ground circuit continuity from switch to PCM or chassis ground
3
Step 3: Disconnect switch and measure signal wire voltage at PCM connector
4
Step 4: Inspect wiring for damage or contact with other power sources
5
Step 5: Test switch operation by grounding signal wire through appropriate resistor and observing PCM response
Estimated Repair Cost
$75 - $425
Parts + labor, varies by vehicle and location
The PCM detects abnormally high voltage on the idle speed selector sensor/switch circuit. The signal voltage exceeds the maximum threshold for any valid switch position.
The most common cause of P256D (Engine Idle Speed Selector Sensor/Switch Circuit High) is: Open circuit in ground path for switch
Typical repair costs for P256D range from $75 to $425, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving. Engine will operate at default idle speed, which may not be optimal for PTO use or cold starts.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P256D to identify the root cause.
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Quick Info
Category
Powertrain
System
Engine Idle Control
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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