40 mph in first gear! That will destroy the engine too. If it's second gear it's stuck in, that's "limp-in" mode that lets you drive it slowly to a repair shop without needing a tow truck. Once it goes to limp-in, there WILL be at least one diagnostic fault code in memory but it's the Transmission Computer that must be checked for codes, not the Engine Computer.
There are a couple of generalizations you can look for that might provide some clues. First, if it starts out in first gear and stays there, something is not working in the valves / solenoid pack. That's pretty rare. If it starts out in second gear right after starting the engine and putting it in drive, it is likely to be sensor-related since the computer detected a problem right away. You will notice lower power than normal when starting to move. If it starts out in first gear like normal, then shifts to second like normal, and maybe even third gear, then suddenly goes back to second and stays there, it still could be a sensor problem but more likely it is due to slippage in one of the clutch packs. When that slippage is detected, the transmission will stay in second gear until the ignition switch is cycled off and the engine is restarted.
If there's a problem inside the transmission, the dealer will usually replace it with a remanufactured one to insure quality and speed of repair. A transmission specialty shop will rebuild yours or one like it that they rebuilt previously. The dealer's transmission will have the latest updated parts to increase reliability. The independent shops know many tricks to increase reliability but will have to charge you extra to replace updated parts if they rebuild your transmission.
Thursday, February 3rd, 2011 AT 8:55 PM