My AEM X-Series temperature gauge shows a wrong or unstable reading in cold weather. Is it faulty?

Not necessarily. The temperature sensor is designed for the normal operating range and generally reads accurately from about 40°C and upwards, so readings on a cold engine in near-zero ambient temperatures can be off. If the reading is still wrong when the engine is at operating temperature, send us the complete kit (gauge and sensor) so we can test them together.