2001 - 2004.5 Allison 1000 Trouble Shooting
The Allison 1000 5 Speed transmission came in 2001 - 2004.5 heavy duty Chevy and GMC trucks. At the time it came out it was cutting edge. Smoother shifting and more robust than almost any other transmission that came from the factory.
However, they do have a reputation for having issues once power and abuse have been added to the truck. Most commonly they go into "Limp Mode" during the 4 to 5 up shift or 5 to 4 downshift under hard acceleration. This will cause a check engine light to come up on your dash and the truck to be limited by power and speed. When this happens the most reported code is P0700. GM defines PO700 as Transmission Control Module Requested MIL Illumination. Simply said, it's a general code for transmission issues and generally is accompanied by another code such as P0701, P0702 and so on.
Many truck owners first reaction is to replace the Transmission Control Module (TCM) as soon as this code pops up. The reality is further diagnosis is needed to tell what the issue is and the best way to resolve it. Never jump to conclusions and always take into consideration the circumstances behind the issue prior to the code comimg up.
Where can you learn more about P0700 codes in your Duramax?