* Fluid Viscosity: This is the most probable cause. The transmission fluid's viscosity (thickness) changes dramatically with temperature. If the wrong type of fluid was used (one that doesn't perform well in cold temperatures), or if the fluid has degraded over time, it could be too thick when cold, causing the slipping. The fluid *might* be thinning out enough once it warms up to function properly.
* Valve Body Issues (Internal): The valve body controls fluid flow within the transmission. A small issue within the valve body might not manifest until the fluid is cold and thicker, preventing proper shifting. This is less likely given the recent rebuild, but not impossible – perhaps a component was slightly faulty or incorrectly installed.
* Solenoid Problems: Solenoids are electrically controlled valves within the transmission. Cold temperatures can impact their performance, leading to shifting issues. Again, this would be surprising given a recent rebuild but is possible.
* Cooler Lines/Fluid Passage Restrictions: If there is a restriction in the transmission cooler lines or within the transmission itself, the cold, thicker fluid might be further hindered in its flow.
* Torque Converter Issues (Less Likely): Though less likely given the specific nature of the problem, a slight issue within the torque converter could manifest under cold conditions.
What to do:
1. Check the Transmission Fluid: This is the *first* step. Check the fluid level *when the car is cold*. Also, note its color and smell. Dark, burnt-smelling fluid suggests degradation. The type of fluid is critical – consult your owner's manual for the correct specification.
2. Consult the Transmission Shop: Since it was recently rebuilt, reaching out to the shop that did the work is crucial. They should have records of the fluid used and the work performed. They might cover the problem under warranty or offer insight based on their prior work.
3. Professional Diagnosis: If the fluid is fine, you need a professional diagnosis from a reputable transmission specialist. They'll have the tools and expertise to determine the exact cause. They can check fluid pressure, solenoid operation, and inspect the valve body for issues.
Avoid driving the car extensively with this problem, as it could lead to further damage to the transmission. The cold-weather-specific nature strongly suggests a fluid or internal component issue related to temperature sensitivity.
