P0738 TCM Engine Speed Output Circuit Low

The P0738 trouble code is an OBD-II diagnostic trouble code related to the Transmission Control Module (TCM) engine speed output circuit.

P0738 – TCM Engine Speed Output Circuit Low

This code means that the TCM has detected an engine-speed output signal that is lower than the expected value.

The engine-speed signal is important for transmission control. The TCM uses engine RPM information to determine appropriate shift timing, gear selection, torque converter clutch operation and other transmission functions.

Depending on the vehicle, engine-speed information may come from the crankshaft position sensor, PCM, a dedicated signal circuit or communication between the PCM and TCM through the CAN network.

P0738 does not necessarily mean that the engine is physically running at low RPM. The word "Low" generally refers to the electrical or data signal being lower than expected.

What Does P0738 Mean?

P0738 can be broken down as:

P = Powertrain
0 = Generic OBD-II code
7 = Transmission-related system
3 = Transmission-specific code group
8 = Specific fault identifier

The important description is:

TCM Engine Speed Output Circuit Low

The TCM is detecting an engine-speed-related output signal below its expected range.

For example, the actual engine may be running at 2,500 RPM while the TCM receives information corresponding to a much lower RPM.

This creates a mismatch between actual engine operation and the information being used for transmission control.

Why Does the TCM Need Engine RPM?

The transmission cannot make accurate decisions without knowing how fast the engine is rotating.

The TCM uses engine-speed information together with:

• Transmission input speed

• Transmission output speed

• Vehicle speed

• Throttle position

• Engine load

• Transmission fluid temperature

• Selected gear

• TCC status

The TCM uses this information to determine:

• When to upshift

• When to downshift

• How to control shift pressure

• When to engage the torque converter clutch

• When to release the TCC

• Whether the transmission is slipping

• Whether the commanded gear has been achieved

If the engine-speed signal is too low or incorrect, the TCM may make incorrect decisions.

What Does "Circuit Low" Mean?

The word "Low" can be misunderstood.

P0738 does not simply mean:

"The engine RPM is too low."

It means that the engine-speed output signal or data received by the TCM is below the expected level.

Depending on the vehicle, this could be caused by:

• Low signal voltage

• Low signal frequency

• Missing signal pulses

• A weak sensor signal

• A short to ground

• Communication problems

• Incorrect RPM information from the PCM

• An internal TCM circuit problem

The exact meaning depends on the vehicle's electrical architecture.

Symptoms of P0738

Possible symptoms include:

• Check Engine Light

• Transmission warning light

• Harsh shifting

• Delayed shifting

• Incorrect gear selection

• Unexpected downshifts

• Unexpected upshifts

• Transmission stuck in one gear

• Limp mode

• Abnormal RPM behavior

• Poor acceleration

• Torque converter clutch problems

• Poor fuel economy

• Transmission slipping

• Reduced drivability

Some vehicles may store P0738 without producing obvious symptoms.

What Does P0738 Feel Like While Driving?

Harsh shifting

If the TCM receives incorrect engine-speed information, it may command shifts at inappropriate times.

Delayed gear changes

The TCM may postpone a shift because the engine-speed data does not make sense.

Unexpected downshifting

Incorrect RPM information can cause the transmission to select a lower gear unnecessarily.

Unexpected upshifting

The opposite can also occur if the TCM believes engine speed is different from reality.

Limp mode

The transmission may enter a protective operating mode.

Abnormal RPM behavior

The dashboard tachometer or scan-tool data may appear inconsistent with actual engine operation.

Torque converter problems

The TCM uses engine RPM when controlling TCC operation, so incorrect RPM information can interfere with torque-converter lockup.

Common Causes of P0738

Short to ground

A signal wire may be unintentionally connected to ground, pulling the signal below its expected level.

Open or damaged wiring

A damaged circuit can cause the engine-speed signal to become weak or disappear.

High resistance in the circuit

Corrosion or damaged wiring can reduce signal quality.

Corroded connector

Poor terminal contact can reduce voltage or signal quality.

Loose terminal

A loose pin can create an intermittent or weak signal.

Faulty engine-speed signal source

The component producing engine-speed information may be defective.

Faulty crankshaft position sensor

On many vehicles, the crankshaft position sensor is a primary source of engine-speed information.

Faulty camshaft position sensor

Some systems use camshaft information together with crankshaft information.

Damaged reluctor or tone wheel

A damaged or incorrectly positioned trigger wheel can produce an abnormal speed signal.

PCM problem

The PCM may calculate or transmit incorrect engine-speed information.

CAN communication problem

If the TCM receives engine-speed information digitally, communication problems can result in incorrect or missing data.

Poor TCM ground

A poor module ground can interfere with signal processing.

Low system voltage

Insufficient electrical voltage can affect sensors and control modules.

TCM internal fault

A failure inside the TCM can affect the engine-speed output circuit.

Is P0738 Serious?

P0738 can be a serious transmission-control problem.

The transmission depends on accurate engine-speed information.

If the TCM cannot determine engine speed correctly, it may not be able to control the transmission normally.

Possible consequences include:

• Harsh shifting

• Incorrect gear selection

• Limp mode

• TCC malfunction

• Increased transmission stress

• Poor fuel economy

The actual severity depends on the cause and the vehicle's response.

Can You Drive With P0738?

If the vehicle is shifting normally and there are no severe symptoms, a short trip to a repair facility may be possible.

However, continued driving is not recommended if the vehicle:

• Enters limp mode

• Shifts violently

• Remains stuck in one gear

• Loses normal gear selection

• Experiences severe hesitation

• Has significant drivability problems

If the transmission is being controlled using incorrect engine-speed information, continued operation can increase transmission stress.

P0738 vs. P0739

These codes are closely related.

P0738 – TCM Engine Speed Output Circuit Low

The TCM detects an engine-speed output signal that is lower than expected.

P0739 – TCM Engine Speed Output Circuit High

The TCM detects an engine-speed output signal that is higher than expected.

The two codes therefore indicate opposite signal conditions.

P0738 vs. P0737

P0737 – TCM Engine Speed Output Circuit

A general engine-speed output circuit fault.

P0738 – TCM Engine Speed Output Circuit Low

A specific low-signal condition.

P0738 vs. P0725

These codes can appear similar but refer to different parts of the engine-speed information path.

P0725 – Engine Speed Input Circuit

Generally concerns the engine-speed input received by the transmission control system.

P0738 – TCM Engine Speed Output Circuit Low

Specifically identifies a low engine-speed output signal associated with the TCM.

The exact circuit arrangement varies by manufacturer.

Where Does the Engine-Speed Signal Come From?

There is no universal design.

Depending on the vehicle, the TCM may receive engine-speed information from:

• Crankshaft position sensor

• Camshaft position sensor

• PCM

• Dedicated engine-speed signal

• CAN communication network

Therefore, the exact diagnostic procedure must be based on the vehicle's wiring diagram and manufacturer service information.

How to Diagnose P0738

A proper diagnosis should determine where the engine-speed information becomes incorrect.

Do not immediately replace the TCM.

1. Scan all control modules

Use a professional diagnostic scanner to retrieve codes from:

• PCM

• TCM

• ABS

• Body control modules

• Other networked modules

Pay particular attention to:

• Crankshaft sensor codes

• Camshaft sensor codes

• Engine-speed codes

• Transmission speed-sensor codes

• CAN communication codes

• Low-voltage codes

If other codes are present, diagnose them first when appropriate.

2. Check live engine RPM

Compare:

• Actual engine RPM

• Scan-tool RPM

• PCM RPM

• TCM RPM

• Dashboard tachometer

If the PCM reports 2,500 RPM but the TCM reports 500 RPM, there is a clear discrepancy.

This can help identify where the problem exists.

3. Check freeze-frame data

Review:

• Engine RPM

• Vehicle speed

• Transmission temperature

• Engine load

• Gear

• Battery voltage

• TCC status

Determine exactly when P0738 was stored.

4. Check battery voltage

Verify that the vehicle's electrical system is operating normally.

Check:

• Battery voltage

• Alternator output

• Battery terminals

• Engine grounds

• Chassis grounds

• TCM grounds

• PCM grounds

5. Inspect the wiring

Look for:

• Broken wires

• Chafing

• Short to ground

• High resistance

• Melted insulation

• Pinched wiring

• Poor previous repairs

Pay particular attention to wiring around the engine and transmission.

6. Inspect connectors

Check the relevant connectors for:

• Corrosion

• Loose terminals

• Bent pins

• Water intrusion

• Damaged seals

• Poor terminal tension

7. Check for a short to ground

Because P0738 specifies a low condition, a short to ground is an important possibility.

Test the circuit according to the manufacturer's wiring diagram.

8. Test the engine-speed signal source

If the vehicle uses a dedicated engine-speed sensor, inspect:

• Sensor power

• Sensor ground

• Signal wire

• Connector

• Mounting

• Physical condition

9. Test the crankshaft position sensor

If engine-speed information originates from the crankshaft position sensor, check its output.

An oscilloscope can be especially useful because it allows the technician to see the actual signal waveform.

Look for:

• Missing pulses

• Weak signal

• Irregular waveform

• Intermittent signal loss

• Incorrect frequency

10. Inspect the reluctor or tone wheel

If the crankshaft sensor reads a reluctor wheel, inspect for:

• Damaged teeth

• Missing teeth where not designed

• Excessive runout

• Incorrect positioning

• Mechanical damage

A sensor can be perfectly functional while the trigger mechanism is damaged.

11. Check camshaft position information

If the vehicle uses camshaft data in its engine-speed calculation, verify the camshaft sensor and related wiring.

12. Compare PCM and TCM RPM data

This is one of the most important diagnostic steps.

For example:

Actual engine RPM: 2,500

PCM RPM: 2,500

TCM RPM: 400

This suggests the problem may exist in the information path between the PCM and TCM or inside the TCM.

On the other hand:

Actual engine RPM: 2,500

PCM RPM: 400

TCM RPM: 400

This points more strongly toward the engine-speed signal source or PCM-side problem.

13. Check CAN communication

If engine RPM is transmitted digitally, inspect the communication network.

Check for:

• CAN High problems

• CAN Low problems

• Poor connections

• Communication interruptions

• Network-related diagnostic codes

• Module power and ground problems

14. Check the TCM output circuit

If the vehicle uses a dedicated TCM engine-speed output circuit, test:

• Signal voltage

• Signal frequency

• Continuity

• Resistance

• Short to ground

• Short to power

• Signal waveform

Follow manufacturer specifications.

15. Check TCM power and ground

A poor ground can produce incorrect signal readings.

Voltage-drop testing is preferable to relying solely on visual inspection.

16. Test the TCM

If the signal source, wiring, connectors, power, grounds and communication network are confirmed to be good, the TCM may become a suspect.

Can a Bad Crankshaft Position Sensor Cause P0738?

Yes, depending on the vehicle.

The crankshaft position sensor is one of the most important sources of engine-speed information.

If its signal is weak, missing or incorrect, the PCM may calculate incorrect engine RPM and transmit incorrect information to the TCM.

However, P0738 specifically identifies a low TCM engine-speed output condition, so the complete signal path should be tested before replacing the sensor.

Can a Bad Camshaft Sensor Cause P0738?

Potentially.

Some engine-management systems use camshaft and crankshaft signals together.

A faulty camshaft signal can therefore affect the engine-speed information used by the transmission controller.

Can a Bad TCM Cause P0738?

Yes.

An internal TCM fault can cause an incorrect engine-speed output.

However, TCM replacement should only be considered after the external circuit and signal sources have been verified.

Can a Bad PCM Cause P0738?

Yes, depending on the vehicle architecture.

If the PCM generates or transmits engine-speed information, a PCM problem can result in incorrect information reaching the TCM.

Can a CAN Bus Problem Cause P0738?

Yes.

On vehicles that transmit engine RPM through CAN communication, network problems can result in missing or incorrect engine-speed data.

Can Low Battery Voltage Cause P0738?

It can contribute to the problem.

Low system voltage can affect sensors and control modules.

The battery, alternator and module grounds should therefore be checked during diagnosis.

Can P0738 Cause Transmission Problems?

Yes.

Incorrect engine-speed information can affect:

• Shift timing

• Gear selection

• TCC control

• Line-pressure strategy

• Transmission protection strategies

Can P0738 Cause Limp Mode?

Yes.

If the TCM cannot trust engine-speed information, it may activate a failsafe strategy.

Can P0738 Cause Low Engine RPM?

The code itself does not necessarily mean that the engine is physically running at low RPM.

It means that the TCM is detecting a low engine-speed output signal.

The actual engine RPM should be compared with scan-tool and TCM data.

Can P0738 Cause High RPM?

It can indirectly.

If incorrect engine-speed information causes inappropriate gear selection or prevents normal TCC operation, engine RPM may become abnormal.

Can P0738 Cause Poor Fuel Economy?

Yes.

Incorrect transmission control can prevent the vehicle from operating in its most efficient gear or using proper TCC lockup.

Can P0738 Cause Harsh Shifting?

Yes.

Incorrect RPM information can cause the TCM to make incorrect shift decisions.

Common Diagnostic Mistakes With P0738

Replacing the TCM immediately

The problem may be a sensor, wiring or communication fault.

Replacing the crankshaft sensor without testing

The sensor may be fine while the wiring or trigger wheel is defective.

Assuming "low" means low engine RPM

The code refers to the signal or data being lower than expected.

Ignoring CAN communication

On modern vehicles, the TCM may receive engine RPM through the communication network rather than a simple dedicated wire.

Checking only continuity

A circuit can have continuity and still suffer from high resistance, poor terminal contact or intermittent failure.

Ignoring grounds

Poor PCM or TCM grounding can cause abnormal electronic behavior.

Ignoring charging voltage

Abnormal system voltage can affect control modules and sensors.

Using a generic wiring diagram

The exact engine-speed signal architecture varies significantly between vehicles.

What Is the Correct Repair for P0738?

The repair depends on the confirmed cause.

Possible repairs include:

• Repairing a short to ground

• Repairing damaged wiring

• Replacing damaged connector terminals

• Cleaning connector corrosion

• Repairing module grounds

• Replacing a faulty crankshaft position sensor

• Replacing a faulty camshaft position sensor

• Repairing a damaged reluctor or tone wheel

• Repairing CAN communication wiring

• Correcting low system voltage

• Updating PCM software

• Updating TCM software

• Reprogramming the TCM

• Repairing the TCM output circuit

• Replacing the TCM if its failure is confirmed

• Repairing or replacing the PCM if its failure is confirmed

How to Test the Vehicle After Repair

After the repair, verify that the engine-speed signal is correct throughout the operating range.

Check that:

• Actual engine RPM is correct

• PCM RPM matches actual engine speed

• TCM RPM matches PCM RPM

• Engine-speed signal remains stable

• CAN communication is normal

• Transmission shifts correctly

• Torque converter clutch operation is normal

• Limp mode does not activate

• No additional codes appear

• P0738 does not return

The vehicle should be road-tested through different engine speeds, gears and load conditions.

Frequently Asked Questions About P0738

What does P0738 mean?

P0738 means the TCM has detected a low engine-speed output circuit signal.

Does P0738 mean the engine is running at low RPM?

Not necessarily. "Low" generally refers to the engine-speed signal or data being lower than expected.

Can a crankshaft sensor cause P0738?

Yes, depending on the vehicle's system architecture.

Can a wiring problem cause P0738?

Yes.

Can a short to ground cause P0738?

Yes.

Can a bad connector cause P0738?

Yes.

Can a bad TCM cause P0738?

Yes, but the external circuit should be tested first.

Can a bad PCM cause P0738?

Yes, depending on how engine-speed information is generated and transmitted.

Can a CAN communication problem cause P0738?

Yes, on vehicles that transmit engine-speed information through the CAN network.

Can P0738 cause harsh shifting?

Yes.

Can P0738 cause limp mode?

Yes.

Can P0738 cause poor fuel economy?

Yes, indirectly.

Can P0738 damage the transmission?

Incorrect transmission control can increase transmission stress, so the underlying fault should be diagnosed and repaired promptly.

Can I drive with P0738?

If the vehicle operates normally, a short trip to a repair facility may be possible. If the transmission shifts violently, remains in one gear or enters limp mode, continued driving should be avoided.

Does P0738 mean the transmission needs to be rebuilt?

No. P0738 is an engine-speed signal/output problem and does not automatically indicate internal transmission failure.

Final Thoughts on P0738

P0738 – TCM Engine Speed Output Circuit Low means that the transmission control system is detecting an engine-speed output signal or data value that is lower than expected.

The most important point is that P0738 does not automatically mean low engine RPM.

The technician should compare the actual engine speed with the values reported by the PCM and TCM.

A useful diagnostic sequence is:

Scan All Modules → Check Live RPM → Compare Actual/PCM/TCM RPM → Check Battery Voltage → Inspect Wiring → Inspect Connectors → Test Crankshaft/Camshaft Signal → Check CAN Communication → Test TCM Output → Evaluate PCM/TCM

The fault can originate from a relatively simple wiring or connector problem, a faulty engine-speed sensor, a damaged trigger wheel, a communication problem or, less commonly, a defective control module.

Therefore, P0738 should be diagnosed by following the engine-speed signal path rather than assuming that the transmission itself has failed.