Technical Description of OBD-II Trouble Code
The circuit for the internal temperature sensor of the PCM/ECM/TCM is experiencing low voltage.
What does that mean?
This is a generic DTC that applies to many OBD-II vehicles (1996-newer), including Mazda, Honda, Dodge, Ford, BMW, VW, etc. The exact repair steps may vary depending on the year, make, model, and powertrain configuration.
OBD-II trouble codes P0666, P0667, P0668, and P0669 are associated with the internal temperature sensor circuit of different modules. This circuit includes the PCM, ECM, and/or TCM.
The purpose of the PCM/ECM/TCM internal temperature sensor circuit is to monitor the temperature of various sensors within the control modules. Malfunctions are identified through control module self-test failures. In some vehicles, the three modules are integrated into one unit called the PCM.
When the PCM, ECM, or TCM detects an electrical signal lower than expected within the internal temperature sensor circuit, code P0668 will be set and the check engine light or transmission warning lamp will illuminate.
An example of a TCM transmission control module:
What is the severity of this DTC?
The severity of this code can vary greatly, from just an illuminated check engine light or transmission warning lamp on a vehicle that starts and runs, to a vehicle that stalls or does not start at all. The seriousness of the code depends on the specific nature of the problem.
What are some of the symptoms of the code?
Symptoms of a P0668 trouble code may include:
What are some of the common causes of the code?
Causes for this P0668 code may include:
What are some P0668 troubleshooting steps?
The first step in the troubleshooting process for any malfunction is to research the Technical Service Bulletins (TSB’s) for the specific vehicle by year, model, and power plant. This can save a lot of time by pointing you in the right direction.
The second step is to locate all of the control modules within this circuit and perform a thorough visual inspection to check the associated wiring for obvious defects such as scraping, rubbing, bare wires, or burn spots. This process must also include ground straps and ground wires. Next, check the connectors for security, corrosion, and damaged pins. This process must include the PCM, ECM, and the TCM based on the specific vehicle and control module configuration. The specific tech data for the automobile will assist you with component location and the control module configuration.
Advanced Steps
The advanced steps are vehicle-specific and require the appropriate advanced equipment to perform accurately. These procedures require a digital multimeter and the specific technical references for the vehicle. Specific technical data will include troubleshooting charts and the appropriate sequence to follow for an accurate diagnosis.
Voltage Checks
Specific troubleshooting guidelines must be referenced to determine the voltage ranges required for the various control modules. These references will include the pin numbers and voltage requirements associated with the PCM/ECM/TCM internal temperature sensor/circuit. Most control modules require a reference voltage of approximately 9 volts, but this may vary based on the specific year and model of the vehicle.
If this process identifies the absence of a power source or ground, continuity testing may be required to check the integrity of the wiring, connectors, and other components. Continuity tests should always be performed with the power removed from the circuit, and the normal readings for wiring and connections should be 0 ohms of resistance. Resistance or no continuity indicates faulty wiring that is open or shorted and must be repaired or replaced. A continuity test from the various control modules to the frame will confirm the serviceability level of ground straps and ground wires. The presence of resistance indicates a loose connection or possible corrosion.
What are some common repairs for this code?
Misdiagnosis may result in replacing the PCM, ECM, or TCM when faulty wiring or loose connections are causing this code. Also, on vehicles equipped with two or more control modules, the wrong module may be replaced in error.
Hopefully, the information in this article has been helpful in pointing you in the right direction to correct the problem with your PCM/ECM/TCM internal temperature sensor/circuit fault code. This article is strictly informational, and the specific technical data and service bulletins for your vehicle should always take priority.
