P0887 Transmission Control Module (TCM) Power Relay Control Circuit High

The P0887 – Transmission Control Module (TCM) Power Relay Control Circuit High diagnostic trouble code (DTC) indicates that the transmission control system has detected an abnormally high electrical signal or voltage in the control circuit of the TCM power relay.

The TCM power relay is responsible for supplying electrical power to the transmission control module and, depending on the vehicle, other transmission-related circuits. The relay allows the control module to receive the required power when the ignition is switched on or when the vehicle's control system commands the transmission electronics to operate.

When P0887 is stored, the powertrain control module (PCM), TCM, or another control unit has detected that the relay control circuit voltage is higher than the expected value.

This is primarily an electrical power-supply and relay-control problem. It does not automatically mean that the TCM itself is defective or that the transmission has internal mechanical damage.

What Does P0887 Mean?

The code can be understood as follows:

• P – Powertrain-related fault

• 08 – Transmission control-related system

• 87 – Specific fault identification

• TCM Power Relay – The relay that controls electrical power supplied to the transmission control system

• Control Circuit High – The control circuit voltage is higher than the expected value

In simple terms:

P0887 means that the transmission control system is detecting excessive voltage in the circuit used to control the TCM power relay.

The exact circuit design varies between manufacturers.

What Is the TCM Power Relay?

The Transmission Control Module (TCM) needs a stable electrical supply to operate the transmission.

Instead of routing all of the TCM's operating current directly through the ignition switch or a small control circuit, the vehicle may use a relay.

A simplified operation looks like this:

Ignition/vehicle control system → Relay control circuit → TCM power relay activates → Battery voltage supplied to TCM → Transmission control system operates

The relay essentially acts as an electrically controlled switch.

When the relay is commanded on, it connects the appropriate power supply to the TCM.

If the relay control circuit develops an abnormal high voltage, the control module can set P0887.

What Does "Control Circuit High" Mean?

"High" does not necessarily mean that the entire vehicle electrical system is overcharging.

It specifically means that the voltage detected on the relevant relay control circuit is higher than the value expected by the control module.

Possible reasons include:

• A short to battery voltage

• Incorrect wiring

• Damaged relay-control wiring

• A faulty relay

• Incorrect relay installation

• A control circuit driver problem

• An electrical backfeed

• A charging-system problem

• A control module fault

The exact cause depends on how the manufacturer designed the relay circuit.

Symptoms of P0887

P0887 can produce very different symptoms depending on whether the TCM is still receiving stable power.

Common symptoms include:

Check Engine Light

The malfunction indicator lamp may illuminate.

Transmission warning light

A transmission or drivetrain warning may appear.

Harsh shifting

The transmission may shift harder than normal.

Delayed shifting

Gear changes may become slow or inconsistent.

Transmission stuck in one gear

The transmission may enter a fail-safe strategy.

Loss of certain gears

Some gears may become unavailable.

Transmission remains in Neutral

In severe cases, the TCM may not be able to control gear engagement correctly.

Vehicle will not move

If TCM power is interrupted or the transmission enters a protective mode, the vehicle may lose drive.

No communication with the TCM

A scan tool may be unable to communicate with the transmission control module.

Intermittent transmission operation

The transmission may work normally and then suddenly develop faults.

Multiple transmission codes

Loss or instability of TCM power can cause several unrelated-looking transmission DTCs to appear simultaneously.

Reduced performance

The vehicle may enter limp mode or a reduced-function strategy.

Is P0887 a Serious Problem?

P0887 can be a serious fault.

The TCM requires stable electrical power to control the transmission.

If the power relay control circuit is malfunctioning, the TCM may not operate correctly.

The problem is especially serious if:

• The vehicle cannot select gears

• The transmission is stuck in one gear

• The TCM cannot communicate with the diagnostic scanner

• The vehicle intermittently loses drive

• Limp mode is activated

• Several transmission systems stop functioning simultaneously

If the transmission continues operating normally and the code is only intermittent, the vehicle may still be driveable for diagnosis.

However, P0887 should not be ignored because a power-supply problem can potentially cause a complete loss of transmission control.

Most Common Causes of P0887

Several electrical problems can cause this code.

Short to battery voltage

This is one of the most important possibilities with a circuit-high code.

A relay-control wire may be unintentionally connected to a constant battery-positive source.

Damaged wiring

The control wire may be:

• Chafed

• Melted

• Pinched

• Incorrectly repaired

• Connected to another power circuit

Faulty TCM power relay

The relay may have an internal electrical or mechanical problem.

The relay contacts may also stick or behave incorrectly.

Incorrect relay

Installing a relay with the wrong pin configuration can produce abnormal circuit behavior.

This can happen after previous repairs.

Corroded relay socket

Corrosion or contamination in the relay socket can cause abnormal voltage readings.

Loose relay terminals

Poor terminal contact can cause unstable relay operation.

Electrical backfeed

Voltage from another circuit may enter the relay-control circuit unexpectedly.

Charging-system problem

Excessively high system voltage can affect multiple electrical circuits.

This is less common than a localized wiring problem but should be checked when the battery or alternator is suspected.

Faulty control-module driver

The module responsible for controlling the relay may have an internal driver fault.

Incorrect aftermarket wiring

Remote starters, alarms, accessories, or previous electrical modifications can sometimes interfere with relay circuits.

Can a Bad TCM Cause P0887?

Yes, but it should not be the first assumption.

The TCM may contain the electronic driver that controls the power relay.

If that driver fails internally, the relay control circuit may behave abnormally.

However, before replacing the TCM, technicians should verify:

• Relay operation

• Relay type

• Relay socket

• Control wiring

• Power supply

• Ground

• Short-to-voltage conditions

• Charging voltage

• Connector condition

A wiring problem can easily be mistaken for a faulty TCM.

Can a Bad Relay Cause P0887?

Yes.

A defective relay can cause abnormal voltage or switching behavior.

However, the relay should be tested rather than replaced automatically.

The relay's:

• Coil resistance

• Power supply

• Ground/control side

• Switching contacts

• Output voltage

should be checked according to the manufacturer's specifications.

Can a Blown Fuse Cause P0887?

A blown fuse is not usually the direct cause of a circuit-high code.

A blown fuse generally creates a loss of power or an open circuit.

However, a fuse or power-supply problem can occur together with P0887 and should therefore be inspected.

If the TCM loses power, other transmission codes may also appear.

Can Low Battery Voltage Cause P0887?

Low battery voltage is not normally the direct cause of a TCM power relay control circuit high fault.

However, unstable system voltage can cause unusual electronic behavior.

It is therefore useful to check:

• Battery voltage

• Charging voltage

• Battery terminals

• Main grounds

• Main power connections

If charging voltage is excessively high, the alternator and voltage-regulation system should be investigated.

Can a Bad Alternator Cause P0887?

It can contribute to electrical problems, but it is not one of the first causes to assume.

If the alternator is overcharging, the entire vehicle's electrical system can experience excessive voltage.

This can produce multiple electronic faults, not just P0887.

If P0887 appears together with:

• Battery warning light

• Multiple module faults

• High system-voltage codes

• Unusual lighting behavior

• Battery overheating

• Other electronic problems

the charging system should be tested.

How Is P0887 Diagnosed?

Because P0887 concerns the TCM's power relay control circuit, diagnosis should focus on the relay, control circuit, power supply, ground, and module command.

1. Scan all control modules

Retrieve all stored and pending DTCs.

Record:

• P0887

• Other transmission codes

• Communication codes

• Low/high voltage codes

• TCM internal faults

• Freeze-frame data

Pay close attention to codes indicating loss of TCM communication or power.

2. Check whether the scan tool can communicate with the TCM

If communication with the TCM is impossible, investigate TCM power and ground circuits immediately.

3. Check battery voltage

Verify that the battery voltage is within the manufacturer's specifications.

4. Check charging voltage

Make sure the alternator is not producing excessive voltage.

5. Locate the TCM power relay

Use the vehicle-specific wiring diagram to identify the correct relay.

Do not rely solely on the label printed on the fuse-box cover.

6. Inspect the relay

Check whether the correct relay is installed and whether there are signs of:

• Heat damage

• Corrosion

• Water intrusion

• Burned terminals

• Loose connections

7. Inspect the relay socket

A damaged relay socket can cause abnormal circuit readings.

8. Check the relay control circuit

Determine what voltage is present on the control side and compare it with the manufacturer's specifications.

9. Check for a short to battery voltage

If the control wire has battery voltage when it should not, inspect the wiring for a short to power.

10. Check TCM power and ground

Verify that the TCM receives the correct power and has a reliable ground.

11. Check the TCM control signal

If all external wiring and components are correct, the TCM's relay-control output may need to be tested.

How to Test P0887 With a Multimeter

A multimeter can be very useful for this diagnosis.

The exact pin locations must be obtained from the vehicle's wiring diagram.

Check battery voltage.

Measure the battery with the ignition off and under operating conditions.

Check charging voltage.

Verify that the alternator is not overcharging.

Check relay power.

Identify the relay's power terminal and confirm that the expected voltage is present.

Check the control circuit.

Measure the voltage on the relay-control circuit with the relay commanded on and off.

Check for unwanted battery voltage.

If the control circuit remains at battery voltage when it should be switched off or pulled low, investigate a possible short to power or control-driver problem.

Check relay coil resistance.

Compare the measured resistance with manufacturer specifications.

Check relay output.

Verify that the relay supplies power correctly when commanded.

Why the Wiring Diagram Is Important

P0887 cannot be diagnosed reliably by guessing which relay terminal is which.

Different vehicles use different relay-control strategies.

For example, one system may use:

TCM supplies ground → Relay coil activates

Another may use:

TCM supplies positive voltage → Relay coil activates

Some systems may also use an integrated power-management module.

Therefore, the technician must identify:

• Relay power input

• Relay output

• Relay coil positive

• Relay coil control

• TCM power feed

• TCM ground

from the correct wiring diagram.

What Is the Difference Between Relay Power and Relay Control?

This distinction is very important.

The relay power circuit carries the electrical supply that eventually powers the TCM.

The relay control circuit is the smaller circuit used to activate or deactivate the relay.

P0887 specifically refers to the control circuit being detected as high.

A problem with the control circuit does not necessarily mean the main TCM power feed is over-voltage.

Can a Short to Power Cause P0887?

Yes.

A short to battery voltage is one of the most logical possibilities for a circuit-high fault.

For example:

Normal: TCM controls relay → Control circuit switches as expected

Fault: Damaged control wire contacts battery positive → Control circuit remains high → TCM detects abnormal voltage → P0887 is stored

The wiring should therefore be inspected carefully for contact with power sources.

Can Corrosion Cause P0887?

Yes.

Corrosion can affect relay sockets, connectors, terminals, and wiring.

Depending on the circuit design, corrosion can create:

• Increased resistance

• Unstable voltage

• Unwanted voltage paths

• Intermittent relay activation

Water intrusion around the fuse/relay box should receive particular attention.

Can P0887 Cause Limp Mode?

Yes.

If the TCM cannot reliably control its power supply, the transmission may activate a fail-safe strategy.

Possible results include:

• Fixed gear operation

• Limited gears

• Harsh shifting

• Delayed shifting

• Loss of reverse

• Transmission warning messages

Can P0887 Cause No Communication With the TCM?

Yes, potentially.

If the TCM power relay or its associated circuit is not operating correctly, the TCM may not receive the required electrical supply.

A scan tool may then report that the TCM is offline or cannot be accessed.

However, a communication failure can also have other causes, so the TCM power and ground circuits must be tested directly.

Can P0887 Cause the Vehicle Not to Move?

Yes, in severe cases.

If the TCM loses stable power or cannot properly control the transmission, the vehicle may:

• Remain in Neutral

• Become stuck in a gear

• Lose Drive

• Lose Reverse

• Enter a protective mode

The exact behavior depends on the transmission design.

Common Diagnostic Mistakes

One of the most common mistakes is immediately replacing the TCM.

P0887 is a power relay control circuit fault, so the electrical circuit should be tested first.

Other mistakes include:

Replacing the relay without testing it

The wiring may actually be shorted to power.

Ignoring the relay socket

The relay itself may be good while the socket has damaged terminals.

Assuming "High" means alternator overcharging

The code refers specifically to the monitored relay-control circuit.

Ignoring aftermarket electrical modifications

Accessories or previous wiring repairs can interfere with relay circuits.

Checking only the battery

A good battery does not prove that the relay-control circuit is functioning correctly.

Ignoring TCM grounds

A poor ground can cause abnormal module behavior.

How Is P0887 Repaired?

The repair depends on the confirmed cause.

Possible repairs include:

• Repairing a short to battery voltage

• Repairing damaged wiring

• Replacing damaged wiring

• Repairing relay terminals

• Replacing a damaged relay socket

• Replacing the TCM power relay

• Correcting an incorrect relay installation

• Repairing power or ground connections

• Repairing water or corrosion damage

• Correcting charging-system problems

• Removing incorrect aftermarket wiring

• Updating TCM software where applicable

• Replacing the TCM if its internal relay-control driver is conclusively diagnosed as faulty

The correct repair is based on testing rather than simply replacing the most expensive component.

Does the TCM Need to Be Replaced?

Not necessarily.

A TCM replacement should generally be considered only after the following have been verified:

• Battery voltage

• Charging voltage

• Fuses

• Relay

• Relay socket

• Control wiring

• TCM power supply

• TCM grounds

• Connector condition

• Short-to-power conditions

If all external circuits are correct and the TCM is demonstrably commanding the relay incorrectly, then a TCM fault becomes more likely.

Can You Drive With P0887?

It depends on the symptoms.

If the transmission operates completely normally and the code is intermittent, the vehicle may remain temporarily driveable.

However, caution is recommended because a relay-control fault can potentially develop into a complete TCM power failure.

Driving should be avoided if:

• The transmission is stuck in one gear

• The vehicle cannot select gears

• The TCM cannot communicate with the diagnostic scanner

• The vehicle intermittently loses drive

• Limp mode is active

• Severe or unpredictable shifting occurs

How to Reset P0887

After repairing the electrical problem, P0887 can be cleared with an appropriate diagnostic scanner.

The recommended sequence is:

Diagnose → Repair → Clear DTC → Verify TCM communication → Test transmission → Rescan

If P0887 returns immediately, the control circuit is still experiencing the condition that caused the code.

Disconnecting the battery is not a proper repair method and may erase learned transmission values or other diagnostic information.

Does P0887 Mean the Transmission Needs Replacement?

No.

P0887 is an electrical control-circuit code and does not automatically indicate internal transmission failure.

Before considering major transmission work, check:

• TCM power relay

• Relay socket

• Fuses

• Battery

• Alternator

• Wiring

• Relay control circuit

• TCM power supply

• TCM grounds

• Connectors

• TCM control output

In many cases, the problem can be repaired without opening the transmission.

P0887 vs. P0885, P0886 and P0888

These codes are closely related to the TCM power relay circuit.

P0885 – TCM Power Relay Control Circuit/Open

The control circuit is detected as open or interrupted.

P0886 – TCM Power Relay Control Circuit Range/Performance

The relay control circuit is not operating within its expected parameters.

P0887 – TCM Power Relay Control Circuit High

The control circuit voltage is detected as higher than expected.

P0888 – TCM Power Relay Sense Circuit

The control system has detected a problem involving the TCM power relay's monitored/sense circuit.

This distinction is useful because P0887 should lead the diagnosis toward an abnormally high relay-control signal, rather than automatically assuming that the TCM has failed.

Final Thoughts on P0887

P0887 – Transmission Control Module (TCM) Power Relay Control Circuit High indicates that the transmission control system has detected excessive voltage in the electrical circuit used to control the TCM power relay.

The most likely areas to investigate include a short to battery voltage, damaged wiring, a faulty relay, damaged relay terminals, corrosion, incorrect relay installation, electrical backfeed, charging-system problems, or a fault inside the module controlling the relay.

The code does not automatically mean that the TCM is defective, and it certainly does not by itself prove that the transmission needs to be rebuilt or replaced.

A proper diagnosis should begin with the battery and charging system, followed by the fuse, relay, relay socket, control wiring, TCM power supply, and grounds. The relay-control signal should then be measured according to the vehicle's wiring diagram.

If P0887 is accompanied by loss of TCM communication, limp mode, inability to select gears, or loss of drive, the vehicle should be diagnosed promptly because an unstable TCM power supply can prevent the transmission from operating correctly.