The EGR valve lowers emissions and increases fuel mileage by partially filling the cylinders with inert exhaust gas rather than fresh air and fuel. A valve that's sticking open can cause stalling or rough running at idle when it isn't supposed to be open. It's more common for the black plastic disc on top of the valve to fail from moisture getting inside but at least up through the '99 model year you couldn't buy that part separately. You can find good used ones at the salvage yard. That's that part that will set a fault code. There are two oxygen sensors; one before the catalytic converter and one after. The first one is responsible for fine tuning the fuel / air mixture for the best fuel economy and lowest emissions. The second one monitors the efficiency of the converter to see if it's doing what it is supposed to. Besides the emissions and fuel mileage issues, when you continue driving with the light on, you will never know when a different problem crops up that can turn into something very expensive very quickly. Overheating and destroying the catalytic converter is the most common and expensive problem.
Monday, March 14th, 2011 AT 6:18 PM