Light Off Catalyst
WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2005 Mercury Sable, 2005 Mercury Mountaineer, 2005 Mercury Monterey, 2005 Mercury Montego, and 2005 Mercury Mariner. However, it is still accessible from the selected car via links, so may be relevant.
As the catalyst heats up, converter efficiency rises rapidly. The point at which conversion efficiency exceeds 50% is called catalyst light off. For most catalysts this point occurs at 246°C to 301°C (475°F to 575°F). A fast light catalyst is a 3-way catalyst (TWC) that is located as close to the exhaust manifold as possible. Because the light off catalyst is located close to the exhaust manifold it will light off faster and reduce emissions quicker than the catalyst located under the body. Once the catalyst lights off, the catalyst will quickly reach the maximum conversion efficiency for that catalyst.