P0668 OBD-II Trouble Code: PCM/ECM/TCM Internal Temperature Sensor Circuit Low

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P0668 is a diagnostic trouble code (DTC) for "PCM/ECM/TCM Internal Temperature Sensor Circuit Low". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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P0668 code definition

The P0668 code identifies a problematic and potentially harmful temperature reading from the engine or transmission via either the engine control module, the transmission control module, or the powertrain control module.

The P0668 code is often associated with the P0667 code.

What the P0668 code means

P0667 is the OBD-II generic code for a fault involving the internal temperature of the engine or transmission at the point where the sensor is located. Located within the engine and transmission are circuits that routinely check in on certain factors like heat and voltage at certain points. These relay the date back to the transmission control module and engine control module. In some cases, the information is also relayed to the powertrain control module. When the P0668 code is indicated, then one of the points in the circuit that is keeping track of the temperature inside of the engine or transmission that is getting a reading that is above or below what is required.

What causes the P0668 code?

The P0668 code has a number of common causes:

  • Faulty temperature sensor within the engine or transmission
  • Faulty wiring within the temperature sensor circuits
  • Overheating in the engine
  • Overheating in the transmission
  • Lack of coolant in the cooling system
  • Defective parts in the powertrain cooling systems

What are the symptoms of the P0668 code?

Since there are not many external indicators of excessive temperatures in the engine or transmission, the first indicator of a problem that would prompt the P0668 code is the Check Engine light on the dashboard. Since this trouble code is of a serious nature, the engine may stall once this code appears. The P0668 code is also capable of making the engine unable to start. If the engine remains running, then there may be sputtering or power loss.

How does a mechanic diagnose the P0668 code?

In most cases, the first step in diagnosis is to have a certified technician use an OBD-II scanner to read the trouble codes from the vehicle in real time. In the case of diagnosing a P0668 code, however, there is a high likeliness that there is an electrical problem and the circuits surrounding the affected circuit need to be tested with a voltmeter prior to any further diagnostics.

Once the electrical components surrounding the powertrain control module, engine control module, and transmission control module have been inspected, the trouble code can be reset by the technician. Once the code is reset, the vehicle will be taken for a test drive. If the code does not appear over the course of the drive, then it may have just been an electrical short or other intermittent issue.

If the code does appear during the test drive, then there is an issue that is causing excessive temperatures in the engine or transmission, and this issue could cause serious, catastrophic damage.

Common mistakes when diagnosing the P0668 code

The P0668 code is often misdiagnosed as a temperature problem within the engine or transmission when it is actually an electrical issue occurring within the circuit responsible for detecting the temperature. Failure can happen at the sensor itself, in the circuitry of the sensor, at the control module, or at any of the points in between. Sometimes the problem can be as simple as a loose connection.

How serious is the P0668 code?

The P0668 code has the potential to be extremely serious. In the worst cases, the P0668 code appears when there has been a catastrophic rise in temperature within the engine or transmission. In cases like these, the only viable repairs may involve rebuilding or even replacing the engine or transmission. The P0668 code has the ability to stop the engine and also the ability to render the vehicle unable to start. If this code appears, it should be assessed by a qualified technician as soon as possible.

What repairs can fix the P0668 code?

The most common repairs to address the P0668 code are as follows:

  • Replace faulty powertrain control module
  • Replace faulty engine control module
  • Replace faulty transmission control module
  • Replace faulty wiring harness
  • Repair or replace damaged sensor circuits
  • Replace or rebuild vehicle transmission
  • Replace or rebuild vehicle engine

Additional comments for consideration regarding the P0668 code

Before any parts are replaced, repaired, or rebuilt, all of the electrical connections and components within the affected area should be tested to ensure that the issue is not purely electrical.

Need help with a P0668 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

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37 years of experience
Subaru Forester - Check Engine Light is on - Santa Fe Springs, California
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