Close-Coupled Catalysts and Fast Light-Off

Cold-start emissions are the biggest single contributor to a vehicle's total tailpipe pollution. During the first 60 to 90 seconds after a cold start, the catalytic converter is not hot enough to work. It needs to reach about 500 degrees Fahrenheit before it starts converting pollutants — this is called light-off temperature. Until it lights off, nearly all the raw exhaust goes straight out the tailpipe. Studies show that 70 to 80 percent of a vehicle's total hydrocarbon emissions happen during cold start.

The simplest way to speed up catalyst light-off is to move it as close as possible to the exhaust ports — right at the engine. A close-coupled catalyst is mounted directly to the exhaust manifold or, in engines with integrated exhaust manifolds, built into the cylinder head area. The shorter the distance between the combustion chamber and the catalyst, the hotter the exhaust gas when it reaches the converter, and the faster light-off happens.

Manufacturers use several strategies together to achieve fast light-off. Retarded ignition timing: the ECM delays ignition timing during cold start, which sends hotter exhaust gas to the catalyst at the cost of some fuel efficiency. Secondary air injection: a pump pushes fresh air into the exhaust manifold, which causes unburned fuel in the exhaust to combust and heats the catalyst. Electrically heated catalysts: some newer vehicles (like the Mercedes-Benz C-Class with M254 engine and some Toyota models) have a small electric heating element in the catalyst that pre-heats it before the engine even starts. Post-injection: a small fuel injection during the exhaust stroke puts unburned fuel into the exhaust to generate heat at the catalyst.

Because the catalyst is close to the engine, it runs hotter than an under-floor catalyst. This can accelerate catalyst degradation if there is a fuel delivery problem — a rich condition or misfiring cylinder dumps raw fuel into a close-coupled catalyst that is already running near its thermal limit, and the resulting over-temperature can melt the substrate. Always fix misfire and fuel trim issues quickly on vehicles with close-coupled catalysts to prevent expensive converter failure. Also, close-coupled catalysts are more expensive to replace because they are often integrated with the exhaust manifold as one assembly.