P0430 - Catalyst System Efficiency Below Threshold (Bank 2)
There are no fuel trim numbers for the downstream oxygen sensors. Fuel metering is not their function. The upstream sensors should change between "rich and "lean" about two times per second. If their average indicates a more-rich or more-lean condition, that is when the Engine Computer adjusts the fuel / air mixture, and it does that constantly while you're driving. Those are the short-term fuel trim numbers.
There is a base line the computer starts at when calculating fuel delivery. When it sees it is always making the same short-term adjustments, it gradually moves the amount of correction to the long-term fuel trim. The next time you start the engine, the computer uses those new long-term numbers as the starting point, then it needs to readjust the short-term numbers less. A different way of saying that is the short-term numbers refer to the adjustments needed right now, and the long-term adjustments are the average of what has been needed over time.
The rear oxygen sensors simply look at the exhaust gas after it has been cleaned up by the catalytic converters. If the converter is working, that rear sensor's readings will switch between "rich" and "lean" perhaps once each minute or two. If the converter is not cleaning up the exhaust gas, since no change is taking place, the rear sensor will change state at the same rate as the front sensor. That is how the computer knows to set the "converter efficiency" fault codes. Where many people get confused is those efficiency codes can only be set from the readings from properly-working oxygen sensors. Replacing the sensors does not fix the problem.
Catalytic converters do not fail too often on their own. The most common cause for a failure is too much unburned gas is burning inside it, typically from a spark-related misfire, then the catalyst melts or becomes contaminated. The fix for code 430 is to replace the converter, but then I would watch the front oxygen sensor's readings closely to be sure it''s switching properly, and the fuel trim numbers are low. If the short-term numbers are high negative, there is too much unburned hydrocarbons in the exhaust, and the computer is trying to reduce that.
Wednesday, April 12th, 2017 AT 3:52 PM