P0855 — Drive Switch Input Circuit High

The diagnostic trouble code P0855 indicates an issue with the drive switch input circuit crossing a high voltage level, which could be caused by an electrical fault. This code generally appears when the vehicle’s Engine Control Unit (ECU) detects an abnormal signal in the circuit related to the drive switch or selector switch.

Note that labels and terminology may vary between manufacturers. Some OEMs may refer to this circuit using different codes or descriptions, but the core components monitored remain similar.

Fast Facts

System affected: Powertrain / Transmission
Severity: Moderate
Main symptoms: Limited driveability, warning lights, or shifting issues
Driveability: Limited / No

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Manufacturer variations for trouble code P0855

Manufacturers may assign different labels or code descriptions for similar circuit issues. The specific diagnostic label for P0855 can vary depending on the vehicle brand and model.

Ford: P0855 — « Drive Switch Input Circuit High »
GM (Chevrolet, GMC, Cadillac): Similar codes within the P0850–P0857 range, often referring to transmission or drive switch circuits.
Volkswagen Group: Sometimes referenced under different codes in their-specific diagnostic systems for gear selection circuits.
Others: Many OEMs may have unique codes, but all relate to drive switch input circuit diagnostics.

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What does trouble code P0855 mean?

This code is triggered when the ECU monitors a high voltage level in the drive switch input circuit, which is typically involved in gear selection or transmission control. The drive switch, often part of the gear shift assembly or drive mode selector, sends signals to the ECU indicating the chosen drive mode (e.g., Park, Reverse, Drive).

The ECU expects the input signal to stay within certain voltage ranges. When it detects a voltage that is too high, it interprets this as a fault, setting the P0855 code.

This situation can be caused by a faulty drive switch, wiring issues, or problems within the ECU itself. The code usually appears in scenarios where there is a concern with transmission control, gear engagement, or drive mode signals.

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Severity and risks of trouble code P0855

The severity of P0855 can vary based on how the circuit fault affects vehicle operation. Typically, it should be considered a moderate issue because it impacts the transmission or drive mode signals, which are essential for safe and correct vehicle operation.

If left unaddressed, this fault could lead to shift irregularities, inability to select certain gears, or transmission malfunctions. Additionally, warning lights such as the transmission or check engine light may illuminate.

In most cases, the vehicle may be driven with caution, but it is recommended to have this problem diagnosed and repaired promptly to avoid further transmission damage or safety issues.

Can the vehicle be driven? Limited — yes, but with caution and professional attention recommended.

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Symptoms of trouble code P0855

The appearance of the P0855 code is often accompanied by certain observable symptoms, although these can vary depending on the vehicle and the severity of the fault.

Warning lights: Transmission warning, check engine, or other dashboard indicator lights may illuminate.
Transmission issues: Difficulty shifting, transmission stuck in a single gear, or erratic shifting behavior.
Drive mode erratic behavior: In vehicles with drive mode select systems, switching may be unresponsive or behave unpredictably.
Limited driveability: Reduced power, stalling, or sluggish response during acceleration.

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Most likely causes of trouble code P0855

Understanding the root causes of P0855 can help target repairs effectively. The causes are generally linked to electrical or sensor faults.

Damaged or faulty drive switch or shift selector switch: The component responsible for sending drive mode signals might be defective or worn out.
Wiring or connector issues: Corrosion, damaged wiring, or loose connectors in the drive switch circuit can cause false high signals.
Short circuit or open circuit in wiring: Wiring faults that create a high-voltage condition or break the circuit entirely.
ECU or transmission control module issue: Although less common, internal faults within the ECU or transmission module can trigger incorrect readings.
Battery or electrical system problems: Weak or fluctuating electrical supply can cause inconsistent sensor signals.

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How to diagnose trouble code P0855

Diagnosing P0855 involves systematic checks to confirm the fault and identify its cause without risking further damage.

Visual inspection: Check wiring harnesses, connectors, and the drive switch for damage, corrosion, or loose connections.
Live data reading: Use a diagnostic scanner to verify voltage levels in the circuit when the ignition is activated.
Test the drive switch or shift selector switch: Use a multimeter or oscilloscope to check if the switch gives correct signals and voltages.
Inspect wiring continuity: Perform resistance or continuity tests on wires related to the drive switch circuitry.
ECU communication check: Confirm that the ECU receives correct signals through factory-specific diagnostic tools, if available.

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Possible repairs for trouble code P0855

Addressing P0855 requires careful identification of the specific fault, with repairs ranging from simple to complex. The following options are typically suggested:

Replace the drive switch or gear selector switch—A common fix if the switch is physically damaged or defective. Professional repair recommended.
Repair or replace damaged wiring or connectors: Fixing corrosion, reconnecting loose connectors, or replacing damaged wiring is essential. Basic repair, often DIY or professional.
Clear short circuits or open circuits in wiring: Repair wiring harness faults identified during diagnosis. Professional repair recommended.
ECU reprogramming or replacement: Only if internal faults are confirmed, which is rare; requires specialised tools and expertise.
Electrical system check and repair: Ensure battery and alternator are functioning properly to maintain electrical stability.

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Vehicles commonly associated with trouble code P0855 in Europe

P0855 is most frequently encountered in vehicles equipped with automatic transmissions that utilize electronic drive mode switches and gear selectors. While not exclusive to any specific brand, some models show higher incidences.

Typical manufacturers include:

Ford: Models with Powershift or similar transmissions
General Motors (GM): Vehicles with electronically controlled transmissions
Volkswagen Group: Certain DSG or dual-clutch systems
Others: Various brands with electronic drive mode controls

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Frequent mistakes with trouble code P0855

Diagnosing or repairing P0855 can sometimes lead to errors that hinder proper resolution. Understanding common mistakes can help technicians avoid unnecessary setbacks.

Neglecting to perform a thorough wiring inspection: Ignoring wiring damage often leads to recurring faults.
Replacing the switch without verifying the circuit: Blind replacement may not resolve underlying wiring issues.
Overlooking the impact of electrical system health: Poor battery or alternator performance can cause false readings.
Misinterpreting data from diagnostic tools: Relying solely on scanner readings without physical inspection can lead to misdiagnosis.

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Quick FAQ — trouble code P0855

Can this code disappear on its own? Not usually. Once set, the code persists until the underlying fault is repaired and the ECU’s fault memory is cleared.

Can I keep driving? Limited — the vehicle might be drivable, but there could be transmission performance issues. A professional diagnosis is strongly recommended to prevent further damage.

Why does the code return after clearing it? Because the fault condition still exists—for example, a wiring issue or a faulty switch—that causes the ECU to re-set the code after it is cleared.