P0668 — PCM/ECM/TCM Internal Temperature Sensor “A” Circuit Low #

Note: The code description may vary depending on the vehicle manufacturer.

• PCM Internal Temperature Sensor “A” Circuit Low (General, GM, Ford, Chrysler)
• ECM Internal Temperature Sensor “A” Circuit Low (Toyota, Honda, Nissan)
• TCM Internal Temperature Sensor “A” Circuit Low (Hyundai, Kia, Mazda)

Summary of code P0668 #

P0668 is an OBD-II diagnostic trouble code indicating a low voltage condition in the internal temperature sensor circuit “A” within a control module. This circuit is integrated into one of the main computers managing your vehicle’s engine or transmission—the PCM (Powertrain Control Module), ECM (Engine Control Module), or TCM (Transmission Control Module). The temperature sensor monitors the module’s internal operating temperature, ensuring it stays within a safe range. If the module detects a voltage lower than expected from this sensor, it stores the P0668 code. This issue can be a sign of electrical faults or even internal module failure, potentially leading to drivability problems.

Common symptoms for code P0668 #

When the control module’s internal temperature reading is abnormally low, your vehicle may behave differently. In some cases, you might not notice immediate issues, especially if the problem is intermittent or minor. However, prolonged operation with this code can trigger a range of symptoms, including:

• Check Engine Light illuminated on the dashboard
• Possible reduced engine or transmission performance (limp mode)
• Hard starting or no start condition
• Erratic shifting (for automatic transmissions)
• Intermittent stalling or hesitation
• Unpredictable engine or transmission operation

Probable causes for code P0668 (by frequency) #

The P0668 code is usually triggered by issues affecting the module’s ability to accurately read its internal temperature. These can be as simple as a wiring concern or as complex as a failed control module. The most common causes are:

1. Faulty PCM/ECM/TCM internal temperature sensor (sensor is built into the module)
2. Short to ground or open circuit in the sensor’s signal wiring
3. Poor electrical connections at module connectors (corrosion, loose pins, bent terminals)
4. Water intrusion or contamination in the control module or harness
5. Defective PCM/ECM/TCM (internal failure, rare but possible)
6. Faulty power or ground supply to the module

Step-by-step diagnostic guide for code P0668 #

Diagnosing P0668 requires a systematic approach, starting with the basics and working towards more complex checks if needed. Here are the recommended steps:

1. Use a professional scan tool to confirm the P0668 code and check for related codes.
2. Inspect the battery voltage and charging system for abnormalities.
3. Visually inspect the PCM/ECM/TCM connectors and harness for signs of corrosion, damage, or water intrusion.
4. Check for loose, bent, or pushed-out pins in the module connectors.
5. Test the continuity and integrity of the sensor’s signal wire (refer to OEM wiring diagrams).
6. If accessible, monitor the internal temperature sensor reading with a scan tool—compare to ambient temperature.
7. Clear the code and perform a road test to see if the code returns.
8. If all wiring checks out and the code persists, suspect internal module failure and consider module testing or replacement with OEM procedures.

Possible repairs for code P0668 #

Repair strategies for P0668 depend on the root cause. Always address the most probable and accessible causes before considering module replacement. Potential repairs include:

• Repairing or replacing damaged wiring or connectors at the PCM/ECM/TCM
• Cleaning or tightening electrical connectors and terminals
• Correcting water intrusion or contamination issues (seal leaks, dry out module area)
• Replacing blown fuses or restoring power/ground supplies
• Replacing the PCM/ECM/TCM if proven defective (requires OEM programming)

Is it safe to drive with code P0668? #

Driving with code P0668 is generally not recommended. While some vehicles may show minimal symptoms at first, this code signals a potential issue with a critical control module. If the module overheats or malfunctions, you could experience sudden loss of power, shifting problems, or even no-start situations. For your safety and to prevent further damage, have the vehicle diagnosed and repaired as soon as possible.

Vehicles most affected by code P0668 #

P0668 can appear on a wide range of vehicles, especially those with advanced electronic control modules. It’s more commonly reported on models from:

• Chevrolet / GMC / Cadillac (GM)
• Ford / Lincoln
• Chrysler / Dodge / Jeep / RAM
• Toyota / Lexus
• Honda / Acura
• Nissan / Infiniti
• Hyundai / Kia
• Mazda

However, this code can technically occur in any OBD-II compliant vehicle with a PCM, ECM, or TCM featuring an internal temperature sensor circuit.

Frequent mistakes and best practices for code P0668 #

Misdiagnosis is common with P0668 due to its internal nature. Many jump to replacing the module, but this can be costly and unnecessary if the true issue lies elsewhere. To avoid pitfalls:

• Do not replace the PCM/ECM/TCM without confirming all power, ground, and signal circuits are good.
• Always inspect connectors and wiring thoroughly for corrosion or damage before suspecting module failure.
• Verify battery and charging system health—low voltage can trigger false sensor readings.
• Use only OEM-approved diagnostic procedures for module testing or replacement.
• Document all findings and repairs—this can help if further troubleshooting or warranty claims are needed.

Quick FAQ for code P0668 #

Mechanics and vehicle owners often have similar questions about this code. Here are quick answers to the most common:

Q: Can I reset the P0668 code and keep driving?
A: You can clear the code, but if the underlying issue isn’t resolved, it will return. Continued driving risks further damage or sudden performance loss.

Q: Does this code mean my PCM/ECM/TCM is dead?
A: Not necessarily. Many times, the problem is as simple as a bad connection or damaged wiring. Only replace the module if all other causes have been ruled out with proper diagnostics.