Postby smj999smj » Sun Sep 21, 2014 10:53 am
Oxygen sensors have a constantly varying voltage, ranging from 0 volts up to approximately 1.1 volts, cycling up and down as the mixture goes from rich to lean. P0420/430 codes are for catalyst efficiency and in order to test a converter for efficiency, one must be able to graph the oxygen sensor signals on one particular bank and compare the frequency of rich-lean cycling of the pre-cat sensor to that of the post-cat sensor. These has to be tested under a certain condition, usually engine fully warmed up and vehicle driving and engine at a specific RPM range. Conditions for testing will be listed in the factory service manual, along with other diagnostic tests to determine the cause of said trouble codes. As a general rule, the post-cat sensor will cycle approximately half the frequency of the pre-cat sensor if the catalytic converter is operating efficiently. A ratio close of 1:1 would indicate an inefficient catalytic converter, meaning that it is not breaking down the exhaust emissions properly. For those vehicles that use air/fuel ratio sensors instead of front oxygen sensors, which look very similar, the air/fuel ratio sensors operate at a much tighter range, usually 400-600 millivolts and even with a scantool can be difficult to test for catalytic efficiency. Bottom line, upstream catalytic converters failing on R51's are not an uncommon problem. If there are no other codes, the engine is running well and there are no exhaust leaks, chances are pretty good that the cause of a P0420/430 code is a bad upstream catalytic converter. As far as aftermarket converters, it's best to stick with a well known brand, like Bosal or Walker. There have been some issues with lesser quality catalytic converters not fixing the problem or not lasting very long.