With the lack of detail and symptoms, I have to guess it is staying in second gear. That is "limp mode" that allows you to drive it slowly to a repair shop. They will start by reading the stored diagnostic fault codes. As a general rule, if it starts out right away in second gear, it is typically a wiring or sensor-related problem that has been detected by the computer. If it starts out in first gear like normal, then bangs back to second gear during or right after an up-shift, and stays there until you turn off the ignition switch, it is usually due to slippage in the clutch packs indicating the need for a rebuild. Your mechanic can also view the "clutch volume index", (CVI). That is a set of four numbers indicating how much fluid it takes to apply each clutch pack. With that he can determine how much life is left in those clutches.
Friday, February 1st, 2013 AT 4:37 AM