"Limp Home Mode" is a Brick Board colloquium for what happens when the computer does not get a signal from the air mass meter.

The air mass meter uses a pre-heated platinum wire to determine the amount of air that enters the airbox. The resistance and voltage on the wire change in response to the density and temperature of the air coming into the box in order to maintain the wire's temperature. The computer uses the resistance and voltage to figure how much air is coming in and deliver the appropriate amount of fuel.

When the AMM "dies", the computer reverts to a pre-programmed mixture that is very, very rich. The engine will run up to about 2500 rpm, and give you only enough power to do maybe 35 or 40 mph. I suppose the idea is that you can drive the car to a service station or back home. But, if you drive it like this for a long period of time, you risk frying your catalytic converter, and the rich mixture could, in theory, removing the oil coating the cylinder walls.