P0668 PCM/ECM/TCM Internal Temperature Sensor Circuit Low

The P0668 – PCM/ECM/TCM Internal Temperature Sensor Circuit Low diagnostic trouble code (DTC) indicates that the vehicle's control system has detected an abnormally low electrical signal from the internal temperature sensor circuit of the PCM, ECM, or TCM.

This code is different from a conventional engine coolant temperature sensor or transmission temperature sensor fault. P0668 concerns a temperature-sensing circuit associated with the control module itself.

The exact design varies between manufacturers. In some vehicles, the temperature sensor is integrated directly into the control module, while in others the monitoring circuit may be implemented differently.

What Does P0668 Mean?

In simple terms:

The control module has detected a temperature sensor signal that is lower than the expected electrical range.

The control module monitors its internal operating temperature because excessive heat can affect electronic components.

When the signal from the internal temperature sensor falls below the expected threshold, the module can store P0668.

The important word here is "Low."

It describes the electrical signal, not necessarily a low physical temperature.

Therefore, P0668 does not automatically mean that the ECM, PCM, or TCM is physically too cold.

What Is the PCM/ECM/TCM?

The terminology depends on the vehicle.

ECM – Engine Control Module

Primarily manages engine functions such as fuel injection, ignition where applicable, throttle control, emissions, and various sensors.

TCM – Transmission Control Module

Controls automatic transmission functions such as gear selection, shift timing, pressure control, and torque converter operation.

PCM – Powertrain Control Module

Combines engine and transmission control functions into one module.

P0668 can therefore relate to the internal temperature monitoring circuit of whichever applicable module is used by the vehicle.

What Does "Circuit Low" Mean?

The "Low" designation means that the module is detecting an electrical signal below the expected range.

Depending on the sensor design, this may be caused by:

• Short to ground

• Sensor failure

• Wiring problem

• Internal electronic failure

• Incorrect reference voltage

• Connector problem

• Control module malfunction

It does not necessarily mean that the actual module temperature is low.

What Causes P0668?

Possible causes include:

• Failed internal temperature sensor

• Short to ground

• Damaged wiring

• Corroded connector

• Loose terminal

• Poor electrical connection

• Internal PCM/ECM/TCM failure

• Incorrect sensor signal

• Reference-voltage problem

• Control module power or ground problem

• Software or calibration issue

• Excessive moisture inside the module or connector

The actual cause must be determined through testing.

Can the Internal Temperature Sensor Fail?

Yes.

The internal temperature sensor or its associated circuitry can fail.

In many vehicles, this sensor is integrated into the control module's electronics rather than being a separately replaceable component.

If the internal circuit has failed, the repair may require:

• Specialized module repair

• Module reprogramming

• Control module replacement

Can a Short to Ground Cause P0668?

Yes.

A short to ground can pull the sensor signal below its expected voltage range.

This is particularly important when P0668 appears suddenly or immediately after wiring repairs.

Inspect the relevant wiring for:

• Damaged insulation

• Pinched wires

• Chafing

• Corrosion

• Incorrect repairs

• Contact with engine or chassis ground

Can a Bad Connector Cause P0668?

Yes.

A damaged or corroded connector can alter the sensor circuit.

Check for:

• Corroded pins

• Bent terminals

• Loose terminals

• Water intrusion

• Damaged connector locks

• Poor terminal tension

If the control module is located in the engine compartment, environmental exposure can be especially important.

Can a Poor Ground Cause P0668?

Yes, potentially.

A poor control-module ground can cause abnormal voltage readings and interfere with internal sensor circuits.

Check:

• Module ground

• Battery negative connection

• Engine ground strap

• Chassis grounds

• Ground voltage drop

A voltage-drop test under load is more useful than simply checking continuity.

Can Low Battery Voltage Cause P0668?

Possibly.

Unstable or insufficient system voltage can cause control modules to behave abnormally.

Check:

• Battery condition

• Battery voltage

• Cranking voltage

• Charging voltage

• Battery terminals

• Main grounds

If several unrelated electrical codes appear at the same time, system voltage should be checked early in the diagnosis.

Can a Bad Alternator Cause P0668?

Potentially.

A charging-system problem can create unstable electrical conditions.

An alternator that is undercharging may cause low system voltage, while excessive or unstable charging voltage can also disrupt electronic modules.

If P0668 appears with other voltage-related DTCs, inspect the charging system.

Symptoms of P0668

Symptoms depend on the vehicle and the module involved.

Possible symptoms include:

• Check Engine Light

• Transmission warning light

• Reduced engine power

• Limp mode

• Poor throttle response

• Difficult starting

• Intermittent stalling

• Transmission shifting problems

• Multiple warning lights

• Communication problems

In some vehicles, the only obvious symptom may be the Check Engine Light.

Can P0668 Cause Limp Mode?

Yes, it can.

If the control module interprets the internal temperature signal as abnormal, it may activate a protective operating strategy.

Depending on the vehicle, this can result in:

• Reduced engine power

• Limited RPM

• Restricted transmission operation

• Fixed gear selection

• Reduced torque

• Fail-safe operation

The exact response is manufacturer-dependent.

Can P0668 Cause Transmission Problems?

Yes, if the affected module is the TCM or an integrated PCM.

Possible transmission symptoms include:

• Harsh shifting

• Delayed shifts

• Incorrect gear selection

• Transmission stuck in one gear

• Limp mode

• Torque converter problems

If P0668 relates to the ECM only, transmission symptoms may not occur.

Can P0668 Cause Engine Problems?

Yes, depending on the affected module.

If the ECM or PCM is affected, the vehicle may experience:

• Reduced power

• Poor throttle response

• Stalling

• Difficult starting

• Irregular engine operation

However, P0668 itself does not indicate an internal mechanical engine failure.

Can P0668 Cause a No-Start Condition?

It is possible, but it is not the most common symptom.

If an internal control-module fault becomes severe enough to interfere with engine-control functions, starting may become impossible.

If the vehicle has a no-start condition, other systems should also be tested, including:

• Battery

• Starter

• Crankshaft position signal

• Fuel pressure

• Injector operation

• Immobilizer

• ECM communication

How Is P0668 Diagnosed?

A systematic approach is important because P0668 can potentially lead to an unnecessary control-module replacement.

1. Scan all control modules

Do not scan only the engine module.

Check:

• ECM

• PCM

• TCM

• ABS

• Body control module

• Other networked modules

Look for communication and voltage-related codes.

2. Check freeze-frame data

Record the conditions under which P0668 was stored.

Pay attention to:

• Engine temperature

• Vehicle speed

• Engine RPM

• Battery voltage

• Ambient temperature

• Transmission temperature

3. Check battery and charging voltage

Make sure the vehicle's electrical system is stable.

4. Inspect the control-module connector

Look for:

• Corrosion

• Water

• Loose pins

• Bent terminals

• Damaged locking mechanisms

5. Inspect wiring

If the temperature circuit has an accessible external section, check it for shorts, opens, and physical damage.

6. Check module power and ground

Verify proper supply voltage and ground quality.

7. Check live data

If the scanner provides internal module temperature data, compare it with realistic conditions.

8. Check the sensor signal

Determine whether the signal is genuinely low and whether it corresponds to the manufacturer's specified range.

9. Check for software updates

A manufacturer software update may address certain control-module monitoring problems.

10. Evaluate the module

If external circuits, power, ground, and software are correct, internal module failure becomes more likely.

Can a Scan Tool Diagnose P0668?

Yes.

A capable diagnostic scanner can provide valuable information.

Depending on the vehicle, it may display:

• Internal module temperature

• Module voltage

• Sensor values

• Freeze-frame data

• Communication status

• Module status

• Additional manufacturer-specific codes

A basic OBD scanner may only show P0668 without providing enough information to determine the root cause.

Why Is Freeze-Frame Data Important?

Freeze-frame data tells you what the vehicle was doing when the fault was recorded.

For example, if P0668 was stored while:

• Battery voltage was extremely low

• Multiple modules lost communication

• The vehicle was being started

then a power-supply problem becomes more interesting.

If the code appears repeatedly under normal voltage and operating conditions, an internal sensor or module problem becomes more likely.

What If P0668 Appears When the Vehicle Is Cold?

This can provide an important clue.

If the vehicle has been parked for several hours and the reported internal module temperature is implausible, the problem is unlikely to be explained by actual module temperature.

For example, if the vehicle is cold but the control module reports an impossible temperature value, investigate:

• Sensor circuit

• Signal voltage

• Reference voltage

• Short to ground

• Internal module electronics

What If P0668 Appears Only When the Vehicle Is Hot?

A temperature-dependent fault can indicate:

• Heat-related electronic failure

• Actual module overheating

• Wiring expansion

• Connector problems

• Internal circuit failure

Compare live data and freeze-frame information when the fault occurs.

Can Water Damage Cause P0668?

Yes.

Water intrusion can cause electrical problems inside the control-module connector or, in severe cases, inside the module itself.

Possible consequences include:

• Short circuits

• Corrosion

• Low sensor signals

• Communication problems

• Multiple unrelated DTCs

If the vehicle has been exposed to flooding, heavy rain, or engine-bay washing, inspect the control-module area carefully.

Can Software Cause P0668?

Possibly.

The control module's software interprets the internal temperature sensor signal.

A software or calibration problem can potentially result in an incorrect interpretation of the sensor value.

If manufacturer service information identifies a software update for the particular vehicle, reprogramming may be part of the repair.

Is P0668 a Serious Code?

It can be, depending on the underlying cause.

If the fault is an electrical signal problem but the module continues to operate normally, the vehicle may have few symptoms.

However, if the control module is actually developing an internal electronic failure, the problem can progress to:

• Limp mode

• Reduced engine power

• Transmission problems

• Stalling

• Communication failures

• No-start conditions

Therefore, P0668 should not simply be cleared and ignored.

Can You Drive With P0668?

Short-term driving may be possible if the vehicle operates normally, but the fault should be diagnosed as soon as possible.

Avoid continued driving if you experience:

• Sudden loss of power

• Stalling

• Transmission malfunction

• Repeated limp mode

• Multiple warning lights

• No-start conditions

• Electrical burning smell

These symptoms may indicate a more serious electronic or electrical problem.

How Is P0668 Repaired?

The correct repair depends on the confirmed cause.

Possible repairs include:

• Repairing damaged wiring

• Repairing a short to ground

• Replacing damaged terminals

• Cleaning or repairing corroded connectors

• Repairing module power or ground

• Correcting charging-system problems

• Updating control-module software

• Repairing the internal temperature circuit

• Repairing the control module

• Replacing and programming the control module

Does P0668 Mean the ECM/PCM/TCM Must Be Replaced?

No.

This is an important point.

P0668 does not automatically prove that the control module has failed.

Before replacing an expensive module, check:

• Battery voltage

• Charging voltage

• Module power supply

• Module ground

• Connector condition

• Wiring

• Signal circuit

• Actual module temperature

• Live data

• Software/calibration

Only after these checks should internal module failure be considered strongly.

Can the Internal Temperature Sensor Be Replaced Separately?

Often, no.

In many control modules, the temperature sensor is integrated into the circuit board.

If this is the case, replacing the individual sensor may not be a manufacturer-supported repair.

A specialist may be able to repair the module electronically, but replacement and programming may be required depending on the vehicle.

P0668 vs. P0669

These two codes are closely related but describe opposite electrical conditions:

P0668 → PCM/ECM/TCM Internal Temperature Sensor Circuit Low

P0669 → PCM/ECM/TCM Internal Temperature Sensor Circuit High

P0668 indicates that the sensor circuit signal is below the expected range, while P0669 indicates that the signal is above the expected range.

Neither code, by itself, proves that the control module's actual physical temperature is low or high.

Common Diagnostic Mistakes

Assuming P0668 means the ECM is too cold

The code refers to a low electrical signal, not necessarily a low physical temperature.

Replacing the control module immediately

Power, ground, wiring, connectors, and system voltage should be tested first.

Ignoring the battery

Unstable voltage can create electronic control-module faults.

Ignoring the ground

Poor grounds can produce abnormal sensor signals and communication problems.

Ignoring additional DTCs

Other codes may reveal a common electrical or network problem.

Clearing the code without checking freeze-frame data

The conditions under which P0668 appeared can provide valuable diagnostic information.

How Do You Clear P0668?

The correct process is:

Scan all modules → Record all DTCs → Check freeze-frame data → Check battery and charging voltage → Inspect module power and ground → Inspect connectors → Check wiring → Verify sensor signal → Check live data → Check software updates → Repair the confirmed fault → Clear DTCs → Test-drive the vehicle → Rescan

If P0668 returns, the underlying fault has not been resolved.

Final Thoughts on P0668

P0668 – PCM/ECM/TCM Internal Temperature Sensor Circuit Low indicates that the control system has detected an abnormally low electrical signal from the internal temperature sensor circuit associated with the powertrain control module.

Possible causes include a short to ground, damaged wiring, corroded connector, poor electrical connection, unstable system voltage, internal sensor failure, software issue, or internal control-module failure.

The most important point is that P0668 does not automatically mean that the PCM, ECM, or TCM is physically too cold or that the module must be replaced.

A proper diagnostic sequence is:

Check freeze-frame data → Check battery and charging voltage → Check module power and ground → Inspect connectors and wiring → Verify the sensor signal → Check live data → Check software → Evaluate the control module.

If P0668 is accompanied by limp mode, stalling, transmission problems, communication faults, or intermittent no-start conditions, the vehicle should be diagnosed promptly because an underlying control-module problem can eventually affect critical engine or transmission functions.