P256D

Code

Generic P — Powertrain

Engine Idle Speed Selector Sensor/Switch Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 34 EN: 36 RU: 21

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Page language: EN

Causes

Short to battery voltage in the sensor/switch signal or reference circuit
Failed Engine Idle Speed Selector sensor or selector switch
Corroded, bent or damaged connector pins
Pinned or chafed wiring harness shorting to a powered conductor
Poor or lost ground at sensor or ECM
Aftermarket electrical modifications or recent repairs with incorrect wiring

Symptoms

MIL (Check Engine Light) illuminated
High or erratic idle speed
Engine may hunt or surge at idle
Possible limp-home behavior or limited performance
Intermittent starting or stalling at idle

What to check

Record freeze-frame and all stored codes; confirm P256D is current and not historic
Visual inspection of sensor, connector and wiring for damage, corrosion, or pin intrusion
Inspect for recent repairs or aftermarket installs that may have disturbed wiring
Check battery voltage and system charging (very low or very high system voltage can affect readings)
Backprobe the sensor connector with ignition ON and measure signal, reference and ground voltages
Wiggle wiring harness while monitoring signal to reproduce the fault

Signal parameters

Expected: sensor/switch signal typically a low-voltage logic or variable signal (commonly 0–5 V or switched to ground depending on design)
High condition: signal voltage measured near battery voltage (e.g., >5.5 V) or above the specified maximum
Reference (if present): stable reference voltage (commonly ~5 V) from ECM—verify within spec
Ground: less than 1 Ω continuity to chassis/ECM ground; no intermittent open

Diagnostic algorithm

Retrieve and record all codes and freeze-frame data, then clear codes and attempt to re-test to confirm code returns.
Perform a careful visual inspection of the Engine Idle Speed Selector sensor/switch, connector and harness for damage, corrosion, loose pins, or aftermarket splices.
With the ignition ON (engine off), backprobe the sensor connector: measure signal, reference (if present) and ground voltages. Compare to expected ranges. A signal near battery voltage indicates a 'high' condition.
Disconnect the sensor/switch and check if the circuit voltage returns to normal or if the code clears. If removing the sensor removes the high signal, suspect wiring or sensor internal short.
Check continuity and shorts: using an ohmmeter, check for short to battery on the signal wire and for open/shorts to ground. Inspect wiring along harness paths and through firewall/engine mounts for chafing.
Perform a wiggle test on harness and connectors while monitoring the signal for intermittent changes. Use an oscilloscope if available to look for noise or spikes.
Repair or replace damaged wiring, terminals or the sensor as indicated. Replace corroded connectors or apply proper terminal repair.
After repairs, clear codes and perform functional test: run engine to verify idle control and confirm P256D does not return. If code persists after wiring and sensor replacement, evaluate ECM power/ground integrity and consider ECM reflash or replacement per manufacturer guidance.
If ECM replacement is considered, confirm problem is not caused by wiring, external modules, or software before replacing the module.