My 2005 Nissan Altima's "Service Engine Soon" light has turned on twice in the past month. A diagnostic test indicates that my catalytic converter is "failing". If I don't have the converter replaced right away, will it harm my engine? – Mark
P0420 is the generic code that was likely stored in your vehicles computer to indicate that the catalytic converter efficiency is below the manufacturer's specified threshold. The fuel management system constantly makes finite fuel adjustments; the oxygen sensors produce a corresponding voltage that varies according to those finite changes. The fuel management system monitors the converter by using the readings from both the upstream (pre-converter) and downstream (post converter) oxygen sensors.
The sole purpose of the downstream oxygen sensor is to monitor how efficiently the catalytic converter is operating. The computer makes these calculations by observing the fluctuating voltage of the upstream sensor and comparing it to the downstream. If the rear reading is similar to the front, this indicates that the converter is no longer "efficiently" doing its job. The "Service Engine Soon" light is illuminated and the P0420 code is stored.
When the converter first starts showing signs of failure, it will not immediately affect the engine. Left unchecked, however, it will eventually degrade and collapse, plugging the exhaust, which will choke vehicle performance and limit the life of the motor.
Lou Trottier is owner-operator of All About Imports in Mississauga. Have a question about maintenance and repair? E-mail globedrive@globeandmail.com, placing "Lou's Garage" in the subject area.
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