P0891 Transmission Control Module (TCM) Power Relay Sense Circuit High

The P0891 trouble code is an OBD-II diagnostic trouble code related to the Transmission Control Module (TCM) power relay sense circuit.

P0891 – Transmission Control Module (TCM) Power Relay Sense Circuit High

This code means that the TCM has detected a higher-than-expected voltage or signal in the circuit used to monitor the TCM power relay.

The TCM power relay supplies electrical power to the transmission control system. The TCM monitors the relay's output or sense circuit to confirm that the relay is operating correctly.

When the voltage detected by the TCM remains above the expected range, P0891 may be stored.

Possible causes include:

• Faulty TCM power relay

• Short to voltage

• Damaged relay wiring

• Incorrect relay

• Damaged relay socket

• Corroded relay terminals

• Loose connector pins

• Wiring damage

• Poor electrical connection

• Faulty fuse box or relay box

• Poor TCM ground

• Battery or charging-system problem

• Faulty TCM

• Incorrect module programming

• Water intrusion

• Previous wiring repair

The exact circuit configuration varies between vehicles.

What Does P0891 Mean?

P0891 – Transmission Control Module (TCM) Power Relay Sense Circuit High

The TCM needs a stable power supply to control the transmission.

A simplified system can be represented as:

Battery → Fuse → TCM Power Relay → TCM → Transmission Control System

The TCM also monitors the relay's operation through a sense circuit.

If the monitored voltage is higher than the expected value, the TCM can store P0891.

A simplified example is:

Relay sense circuit → Unexpected high voltage → TCM detects high signal → P0891

What Does “Circuit High” Mean?

The word “High” refers to the electrical signal being higher than the expected value.

It does not mean:

• High transmission fluid pressure

• High transmission temperature

• High engine RPM

• High gear

• High transmission fluid level

An electrical high condition may be caused by:

• Short to battery voltage

• Incorrect relay

• Incorrect wiring

• Faulty relay contacts

• Internal circuit fault

• Control-module problem

The exact voltage threshold depends on the vehicle manufacturer.

What Is the TCM Power Relay?

The TCM power relay is an electrically controlled switch that supplies power to the transmission control system.

A simplified sequence is:

Ignition ON → TCM relay activated → TCM receives power → Transmission control system operates

The relay may supply power to:

• TCM

• Transmission solenoids

• Mechatronic unit

• Sensors

• Actuators

The exact configuration depends on the transmission.

The TCM monitors the relay circuit to verify that the expected voltage is present.

Symptoms of P0891

Symptoms can vary depending on how high the circuit voltage is and whether the TCM remains operational.

Possible symptoms include:

• Check Engine Light

• Transmission warning light

• Transmission warning message

• Harsh shifting

• Delayed shifting

• Incorrect gear selection

• Transmission stuck in one gear

• Limp mode

• Loss of transmission functions

• Intermittent transmission problems

• Communication codes

• Difficulty starting in some vehicles

In some cases, the vehicle may continue to drive normally while the code is stored.

Common Causes of P0891

Short to voltage

A relay sense wire may accidentally contact a battery or ignition power source.

This can cause the monitored signal to remain higher than expected.

Incorrect relay

Installing a relay with the wrong electrical configuration can produce an incorrect output or sense signal.

This is especially important if a relay has been replaced recently.

Faulty TCM power relay

A damaged relay may have internally stuck or burned contacts.

This can cause an abnormal output voltage.

Damaged relay socket

The relay socket can have:

• Loose terminals

• Burned contacts

• Corrosion

• Incorrect terminal tension

Damaged wiring

The harness may have insulation damage that allows a signal wire to contact another power circuit.

Corroded connectors

Corrosion can alter circuit resistance and voltage.

Poor electrical connection

Loose terminals can create abnormal voltage readings.

Poor TCM ground

A weak ground can cause abnormal voltage measurements and module operation.

Fuse box or relay box problem

Internal terminal damage or water intrusion can affect relay circuits.

Battery or charging-system problem

Abnormally high system voltage can potentially affect control-module circuits.

Water intrusion

Moisture around the TCM, relay box or connectors can create electrical faults.

Faulty TCM

An internal TCM circuit may incorrectly detect or generate the relay sense signal.

Is P0891 a Serious Transmission Problem?

P0891 can be serious because it involves the electrical power supply monitoring of the TCM.

If the TCM is not receiving or interpreting its power supply correctly, transmission operation may become unreliable.

Possible consequences include:

• Limp mode

• Harsh shifting

• Loss of gear selection

• Transmission communication problems

• Failure to control solenoids or actuators

However, P0891 does not automatically mean that the transmission has internal mechanical damage.

The problem may be limited to the relay, wiring, fuse box, connector or electrical supply.

Can You Drive With P0891?

If the vehicle drives normally and P0891 is the only code, it may be possible to drive carefully to a repair facility.

However, the problem should not be ignored.

Avoid driving if you experience:

• Severe shifting

• Loss of gears

• Transmission stuck in one gear

• Limp mode

• Loss of drive

• Unpredictable gear selection

• Multiple module communication failures

A power-supply problem can become more severe and cause the TCM to stop functioning.

P0891 vs. P0892

These codes are closely related.

P0891 – TCM Power Relay Sense Circuit High

The TCM detects a higher-than-expected signal.

P0892 – TCM Power Relay Sense Circuit Intermittent

The TCM detects an unstable or intermittent signal.

In simple terms:

P0891 → Signal too high

P0892 → Signal is unstable or comes and goes

P0891 vs. P0881

P0881 – TCM Power Input Signal Range/Performance

This code indicates that the TCM power input is outside its expected range or performance.

P0891 – TCM Power Relay Sense Circuit High

This specifically points toward a high signal in the relay sense circuit.

These codes can potentially appear together if there is a problem with the TCM power supply.

P0891 vs. P0882

P0882 – TCM Power Input Signal Low

The TCM detects a low power input signal.

P0891 – TCM Power Relay Sense Circuit High

The TCM detects a high signal in the relay sense circuit.

They describe different electrical conditions.

How to Diagnose P0891

A proper diagnosis should focus on the TCM power relay, its sense circuit and the electrical supply.

1. Scan all control modules

Use a professional diagnostic scanner to check:

• TCM

• ECM/PCM

• ABS module

• Body control module

• Other networked modules

Record all stored and pending codes.

2. Check for related codes

Pay particular attention to:

• TCM power codes

• Communication codes

• Low/high voltage codes

• CAN communication faults

Other codes can help identify the source.

3. Check freeze-frame data

Review:

• Battery voltage

• Engine RPM

• Vehicle speed

• Transmission temperature

• Ignition status

• TCM communication status

4. Identify the correct relay

Verify the correct relay specification using the vehicle's wiring diagram or manufacturer information.

Do not assume that every physically identical relay has the same internal configuration.

5. Inspect the TCM power relay

Look for:

• Burned contacts

• Corrosion

• Heat damage

• Loose fit

• Incorrect relay

6. Inspect the relay socket

Check:

• Terminal tension

• Corrosion

• Burn marks

• Loose terminals

• Water contamination

7. Check the sense circuit

If a dedicated sense wire is used, test it for:

• Short to battery voltage

• Short to another power circuit

• Open circuit

• High resistance

• Incorrect voltage

8. Check relay output voltage

Measure the relay output according to the manufacturer's wiring diagram.

Compare the actual reading with the specified value.

9. Check TCM power supply

Verify that the TCM receives the correct voltage under different operating conditions.

10. Check TCM ground

Inspect and test the TCM ground connection.

A poor ground can create abnormal voltage readings.

11. Check battery and charging voltage

Verify:

• Battery voltage

• Alternator output

• Voltage stability

• Battery terminals

• Main grounds

An abnormal charging voltage should be corrected before diagnosing the TCM.

12. Inspect wiring

Look for:

• Chafing

• Melted insulation

• Pinched wires

• Broken wires

• Incorrect repairs

• Water damage

13. Check for water intrusion

Inspect the:

• Relay box

• Fuse box

• TCM connector

• Transmission connectors

14. Monitor live TCM voltage

Monitor the TCM power and relay sense voltage while:

• Starting the engine

• Idling

• Increasing engine speed

• Operating electrical loads

• Moving the relevant harness

This can reveal voltage behavior that a static test may miss.

15. Evaluate the TCM

If the relay, wiring, power supply, ground and connectors are all correct, the TCM may require further testing.

Why P0891 Can Be Misdiagnosed

P0891 may lead to unnecessary TCM replacement.

For example:

Shorted sense wire → High voltage → P0891

Replacing the TCM would not solve the problem.

Another possibility is:

Incorrect relay → Incorrect circuit behavior → P0891

Again, replacing the transmission or TCM would be unnecessary.

The relay circuit should therefore be tested before expensive components are replaced.

P0891 After Battery Replacement

If P0891 appears after battery replacement, inspect:

• Battery terminals

• Battery voltage

• Main grounds

• TCM power relay

• Relay box

• Fuse box

• TCM connectors

Incorrect battery connections or unstable electrical voltage can create control-module faults.

P0891 After Transmission Repair

If the code appears after transmission work, inspect:

• TCM connectors

• Transmission harness

• Relay box

• Fuse box

• Grounds

• Power connections

A harness may have been moved, stretched or incorrectly routed during transmission repair.

P0891 After TCM Replacement

If P0891 appears after installing a replacement TCM, verify:

• Correct TCM part number

• Correct relay

• Power supply

• Ground

• Wiring

• Connector installation

• Module programming

• Vehicle configuration

A replacement TCM may require programming before it can operate correctly.

Can a Bad Relay Cause P0891?

Yes.

A defective relay can cause an abnormal sense signal.

However, the relay should not be blamed automatically.

The following should also be checked:

• Relay socket

• Sense wiring

• Power supply

• Fuse

• Ground

• Battery voltage

• Charging voltage

• TCM

Can P0891 Cause Transmission Problems?

Yes.

If the TCM detects an abnormal power relay condition, it may limit transmission operation.

Possible symptoms include:

• Harsh shifting

• Delayed gear changes

• Limp mode

• Loss of some gears

• Incorrect gear selection

• Communication problems

The severity depends on whether the TCM continues to receive stable power.

Common Diagnostic Mistakes With P0891

Common mistakes include:

• Replacing the TCM immediately

• Replacing the transmission

• Replacing the relay without checking its specification

• Ignoring the relay socket

• Not checking for a short to voltage

• Ignoring TCM grounds

• Not checking charging voltage

• Failing to inspect wiring

• Ignoring water intrusion

• Not checking related communication codes

• Assuming the transmission has internal damage

What Is the Correct Repair for P0891?

The correct repair depends on the confirmed cause.

Possible repairs include:

• Replacing the correct TCM power relay

• Repairing a short to voltage

• Repairing damaged wiring

• Replacing damaged relay terminals

• Repairing the relay socket

• Cleaning corroded connectors

• Replacing damaged connectors

• Repairing TCM power connections

• Repairing TCM ground connections

• Correcting charging-system problems

• Repairing water intrusion

• Replacing the TCM if confirmed faulty

• Correctly programming or configuring the TCM

After the repair, clear the codes and perform a complete transmission road test.

Final Thoughts on P0891

P0891 – Transmission Control Module (TCM) Power Relay Sense Circuit High indicates that the TCM has detected a higher-than-expected electrical signal in the circuit used to monitor the TCM power relay.

The main areas to investigate are the TCM power relay, relay socket, sense circuit, wiring, fuse, power supply, grounds, battery, charging system and TCM.

Because the code concerns the TCM's electrical supply monitoring, it should not automatically be interpreted as an internal transmission failure.

P0891 does not automatically mean that the TCM or transmission needs to be replaced. A short to voltage, incorrect relay, damaged relay socket, wiring problem, poor connector, abnormal charging voltage or poor ground can cause the code. The complete relay and power-supply circuit should be tested before replacing expensive components.