Fault Family System: Powertrain (P-Codes) 5 SAE Codes

Idle Speed Selector Switch

Diagnostic & Technical Explanation

Code P256A (Engine Idle Speed Selector Sensor/Switch Circuit/Open) sets when the ECM sees an open, shorted, implausible, or intermittent signal from the driver's idle-speed selector switch or dial, used on work trucks to raise idle RPM for cab heating, battery charging, or auxiliary equipment. Because the selector often lives on an upfitter/body-builder circuit, an incorrectly wired auxiliary harness is a suspect alongside worn selector contacts and a broken wire at the dash connector.

These 5 diagnostic trouble codes share one detection mechanism and one repair path, which is why MechanicDB groups them into a single authored fault family rather than repeating the same explanation 5 times.

Codes in this family (5)

Code	Definition	Repair coverage
P256A	Engine Idle Speed Selector Sensor/Switch Circuit/Open	3 ranked fixes
P256B	Engine Idle Speed Selector Sensor/Switch Range/Performance	3 ranked fixes
P256C	Engine Idle Speed Selector Sensor/Switch Circuit Low	3 ranked fixes
P256D	Engine Idle Speed Selector Sensor/Switch Circuit High	3 ranked fixes
P256E	Engine Idle Speed Selector Sensor/Switch Circuit Intermittent/Erratic	3 ranked fixes

What this page does not show

This page publishes the fault mechanism and the code index. The licensed dataset adds, for every one of the 15,886 codes, the full authored repair set (56,729 procedures in total) with the fields deliberately withheld here:

Procedures 2..n after the first-listed procedure
Aftermarket parts cost ranges in USD (est_parts_cost_min/max_usd)
Professional labor-hour estimates (est_labor_hours)
Step-by-step diagnostic and repair instructions
Replacement-part mappings and catalog lookups
6,637 manufacturer-specific OEM codes across 32 makes ($149 tier)

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