P0855 Drive Switch Input Circuit High

The P0855 trouble code is an OBD-II diagnostic trouble code related to the Drive switch input circuit.

P0855 – Drive Switch Input Circuit High

This code means that the PCM or TCM has detected a higher-than-expected electrical signal from the Drive switch input circuit.

The Drive switch is part of the vehicle's gear-selection system. Depending on the vehicle, it may be integrated into the gear selector, transmission range switch, shift assembly, or an electronic shift-by-wire system.

When the control module detects an abnormally high signal from the circuit used to identify the Drive (D) position, it can store P0855.

Possible causes include:

• Faulty Drive switch

• Faulty transmission range sensor

• Defective gear selector

• Short to battery voltage

• Damaged wiring

• Incorrect wiring

• Open circuit

• Corroded connector

• Loose connector terminal

• Poor electrical connection

• Incorrect reference voltage

• Faulty TCM

• Faulty PCM

• Poor module ground

• Water intrusion

• Previous wiring repair

• Shift-by-wire system fault

The exact design of the Drive switch circuit varies significantly between manufacturers.

What Does P0855 Mean?

P0855 – Drive Switch Input Circuit High

The transmission control system needs to know which gear the driver has selected.

When the driver selects D, the appropriate switch or sensor provides information to the transmission control system.

A simplified system looks like:

Gear Selector → Drive Switch/Range Sensor → TCM/PCM → Transmission

If the electrical signal associated with the Drive position is higher than the expected value, the control module can store P0855.

The important point is that P0855 is primarily an electrical input-circuit fault.

It does not automatically mean that the transmission has an internal mechanical failure.

What Does “Circuit High” Mean?

The word “High” refers to the electrical signal being higher than the control module expects.

It does not mean:

• High transmission fluid pressure

• High engine RPM

• High vehicle speed

• High transmission temperature

• High gear

A circuit can become electrically high because of:

• Short to battery voltage

• Incorrect wiring

• Faulty switch

• Incorrect reference voltage

• Damaged connector

• Control-module fault

Therefore, the actual circuit voltage should be measured before replacing parts.

What Is the Drive Switch?

The Drive switch is used to communicate the selected D position to the transmission control system.

Depending on the vehicle, this function may be performed by:

• A dedicated switch

• Transmission range sensor

• Gear-position sensor

• Multifunction transmission switch

• Electronic gear selector

• Shift-by-wire module

Older transmissions may use a mechanical selector connected to a range switch.

Newer vehicles can use electronic gear selectors where the driver's input is transmitted electronically to the TCM.

Symptoms of P0855

Possible symptoms include:

• Check Engine Light

• Transmission warning light

• Incorrect gear indication

• D not displayed correctly

• Vehicle fails to recognize Drive

• Vehicle starts only intermittently

• Delayed Drive engagement

• Transmission stuck in Park

• Transmission stuck in Neutral

• Harsh shifting

• Incorrect gear selection

• Limp mode

• Reduced vehicle performance

• Transmission warning message

• Cruise control becoming unavailable

Some vehicles may continue operating normally despite storing P0855.

Common Causes of P0855

Faulty Drive switch

The switch may internally fail and produce an electrical signal that is too high.

Faulty transmission range sensor

A range sensor can provide incorrect gear-position information.

If the sensor incorrectly reports the Drive position or produces an abnormal voltage, P0855 may be stored.

Short to battery voltage

A signal wire can contact a powered circuit.

For example:

Signal wire damaged → Contacts battery/ignition voltage → Signal becomes too high → P0855

Damaged wiring

Wiring can become damaged due to:

• Heat

• Vibration

• Chafing

• Pinching

• Corrosion

• Previous repairs

Corroded connector

Corrosion can alter the electrical characteristics of the circuit.

Loose terminal

A loose terminal can create intermittent or abnormal voltage readings.

Incorrect reference voltage

If the circuit uses a reference voltage, an abnormal reference can make the input signal appear too high.

Poor ground

A poor ground can affect the voltage measured by the control module.

Water intrusion

Water entering the gear selector or transmission connector can cause corrosion and abnormal signals.

Faulty TCM

The TCM may incorrectly interpret the Drive switch signal or have an internal electrical problem.

Faulty PCM

In some vehicle architectures, the PCM participates in gear-position monitoring.

Is P0855 a Serious Problem?

P0855 can be serious because it can affect the vehicle's ability to correctly identify the selected gear.

If the vehicle cannot reliably recognize Drive, it may:

• Refuse to move

• Enter limp mode

• Delay Drive engagement

• Select an incorrect operating strategy

• Prevent normal starting

However, P0855 does not automatically mean that the transmission itself is damaged.

A faulty switch, sensor, connector or wiring can cause the code.

Can You Drive With P0855?

Driving depends on the symptoms.

If the vehicle correctly selects Drive and shifts normally, it may be possible to drive carefully to a repair facility.

However, driving should be avoided if:

• Drive is not recognized

• The gear indicator is incorrect

• The transmission shifts unpredictably

• The vehicle enters limp mode

• Gear engagement is severely delayed

• The vehicle unexpectedly moves

A gear-position problem can create a safety risk.

P0855 vs. P0856

These codes concern different circuits.

P0855 – Drive Switch Input Circuit High

The Drive switch input signal is higher than expected.

P0856 – Traction Control Input Signal Malfunction

The traction-control input signal is malfunctioning.

Although both are electrical input codes, they concern different vehicle systems.

P0855 vs. P0854

The related code P0854 generally indicates an intermittent Drive switch input signal.

P0855 specifically indicates a high input signal.

In simple terms:

P0854 → Drive switch input intermittent

P0855 → Drive switch input high

P0855 vs. Transmission Range Sensor Codes

P0855 can involve the transmission range sensor depending on the vehicle design.

However, not every vehicle uses the same component or circuit.

Some vehicles have:

Gear selector → Range sensor → TCM

Others may use:

Electronic selector → Shift module → TCM

Therefore, the vehicle-specific wiring diagram and service information are important.

How to Diagnose P0855

1. Scan all control modules

Use a diagnostic scanner to check:

• PCM

• TCM

• Body control module

• Gear shift module

• ABS module

Look for additional codes.

Other transmission-range or communication codes can provide important clues.

2. Check the gear indicator

Move the selector through:

P → R → N → D

Observe whether the dashboard displays the correct position.

If the indicator does not correspond to the actual selector position, investigate the range-sensing system.

3. Check battery voltage

Verify:

• Battery condition

• Battery terminals

• Charging voltage

• Main grounds

Unstable voltage can cause electronic transmission problems.

4. Identify the Drive switch circuit

Use the vehicle-specific wiring diagram to identify:

• Power supply

• Ground

• Signal wire

• Reference voltage

• TCM/PCM connection

5. Inspect the switch or range sensor

Check for physical damage and verify that the component responds correctly to gear selection.

6. Measure the signal voltage

Compare the Drive switch signal with the manufacturer's specified voltage.

If the voltage is too high, determine why.

7. Check for a short to voltage

Inspect the signal wire for unwanted contact with:

• Battery voltage

• Ignition voltage

• Another powered circuit

8. Check the wiring

Inspect for:

• Broken insulation

• Chafing

• Pinched wires

• Melted sections

• Corrosion

• Previous repairs

9. Inspect connectors

Check for:

• Corrosion

• Bent pins

• Loose terminals

• Water

• Damaged locking mechanisms

10. Check the ground

Verify the switch, sensor and module grounds.

A poor ground can produce abnormal voltage readings.

11. Monitor transmission live data

Use a diagnostic scanner to observe:

• Selected gear

• Actual gear

• Drive switch status

• Transmission range

• Gear selector position

Move the selector and check whether the data changes correctly.

12. Compare commanded and actual gear

If the selector indicates D but the TCM reports a different position, investigate the range input circuit.

13. Perform a wiggle test

Monitor the Drive input while gently moving the wiring harness.

If the signal changes unexpectedly, an intermittent wiring or connector problem may exist.

14. Inspect for water intrusion

Check the selector and transmission connectors for moisture or corrosion.

Water can cause abnormal electrical signals.

15. Check module communication

Verify communication between:

• Gear shift module

• TCM

• PCM

• Body control module

A communication problem can create incorrect gear-position information.

16. Check aftermarket electrical modifications

Poorly installed accessories can interfere with the gear-selection circuit.

17. Test the TCM/PCM

If the switch, sensor, wiring, connectors, power and grounds are correct, the relevant control module may require further testing.

Why P0855 Can Be Difficult to Diagnose

P0855 may be caused by a simple switch problem, but it can also result from wiring or control-module issues.

For example:

Damaged wire → Short to battery voltage → High input → P0855

Another possibility:

Faulty range sensor → Incorrect voltage → TCM interprets signal as high → P0855

Replacing the transmission without testing these circuits would be unnecessary.

Can a Bad Transmission Range Sensor Cause P0855?

Yes.

If the vehicle uses a transmission range sensor to identify the selected gear, a sensor failure can cause an abnormal Drive input signal.

However, the sensor should be tested before replacement.

Can a Bad Gear Selector Cause P0855?

Yes.

On vehicles with electronic gear selectors, an internal selector or shift-module failure can cause an incorrect Drive signal.

Can Wiring Cause P0855?

Yes.

A short to battery voltage is especially important because the code specifically identifies a high input signal.

Damaged wiring and connector problems should therefore be inspected carefully.

Can P0855 Prevent the Vehicle From Starting?

Yes, depending on the vehicle.

Modern vehicles often require the control system to confirm that the transmission is in Park or Neutral before allowing the engine to start.

If the gear-position information is incorrect, the vehicle may not recognize the correct starting condition.

Can P0855 Cause Limp Mode?

Yes.

If the TCM cannot reliably determine the selected gear, it may activate a protective transmission strategy.

The transmission may then operate with limited gears or altered shift behavior.

Can P0855 Cause Harsh Shifting?

It can.

Incorrect gear-position information can affect the transmission's control strategy.

Possible symptoms include:

• Harsh engagement

• Delayed shifting

• Incorrect shift timing

• Limited gear selection

Can P0855 Damage the Transmission?

P0855 itself does not prove internal transmission damage.

However, continuing to drive while the transmission is selecting or controlling gears incorrectly can potentially create additional mechanical stress.

The electrical problem should therefore be diagnosed promptly.

Common Diagnostic Mistakes With P0855

Common mistakes include:

• Replacing the transmission immediately

• Replacing the range sensor without testing the circuit

• Replacing the gear selector without checking wiring

• Ignoring battery voltage

• Ignoring grounds

• Not checking for a short to voltage

• Ignoring connector corrosion

• Not checking the gear indicator

• Not monitoring live transmission data

• Replacing the TCM without verifying power and communication circuits

What Is the Correct Repair for P0855?

The correct repair depends on the confirmed cause.

Possible repairs include:

• Replacing a faulty Drive switch

• Replacing a faulty transmission range sensor

• Repairing damaged wiring

• Repairing a short to battery voltage

• Repairing an open circuit

• Repairing high-resistance connections

• Replacing damaged connector terminals

• Cleaning corroded connectors

• Repairing water-damaged wiring

• Repairing a gear selector

• Repairing a shift-by-wire module

• Correcting reference-voltage problems

• Repairing module grounds

• Repairing CAN communication problems

• Programming or calibrating a replacement module

• Replacing the TCM/PCM if confirmed faulty

After the repair, clear the diagnostic codes and perform a complete gear-selection and road test.

Final Thoughts on P0855

P0855 – Drive Switch Input Circuit High indicates that the transmission control system has detected a higher-than-expected electrical signal in the Drive switch input circuit.

The most important areas to inspect are the Drive switch, transmission range sensor, gear selector, wiring, connectors, reference voltage, grounds, TCM and PCM.

The key point is that P0855 does not automatically mean that the transmission has an internal mechanical failure. A short to voltage, faulty switch, damaged wiring, corroded connector or range-sensor problem can all produce the code.

A proper diagnosis should begin by identifying the exact Drive-position circuit used by the vehicle and measuring its signal under different gear-selector positions. The technician should then compare the actual signal with the manufacturer's specifications before replacing the switch, range sensor, TCM or any other expensive component.