P0668 indicates that the engine control module (PCM) has detected a low voltage condition from its internal temperature sensor. This sensor monitors the PCM's own operating temperature, and a low reading typically points to a fault within the module itself or its power/ground supply.
The internal temperature sensor or related circuitry within the PCM has failed, causing a low voltage reading. This is the most common cause.
A weak battery or failing alternator can cause the PCM to see lower-than-normal voltages, which may be interpreted as a low sensor signal.
A corroded or loose ground for the PCM can result in voltage drops that mimic a low sensor reading.
A blown fuse, damaged relay, or wiring fault in the PCM's main power feed can cause low voltage to the module.
Corrosion, bent pins, or poor contact at the PCM harness connector can introduce resistance and lower the sensor voltage.
It means the PCM's internal temperature sensor is reporting a voltage lower than the expected range. This sensor is built into the PCM and helps the module manage its own temperature.
In most cases, the vehicle will still run, but the Check Engine Light will be on. If the PCM is failing, it could eventually cause drivability issues or prevent the engine from starting. It's best to diagnose and repair promptly.
No. While a failed PCM is the most common cause, you should first check the battery, charging system, PCM grounds, and power supply circuits. If those are fine, then PCM replacement is likely needed.
Yes, a poor PCM ground can cause voltage drops that trigger this code. Always inspect and clean ground connections before replacing the PCM.