Originally Posted by Duke2.6

"If the emission system is performing optimally the O2 content should be zero or no more than 0.1 percent, and CO2 should be over 15 percent because all the O2 has been consumed oxidizing HC to H2O and CO2. O2 content is a key piece of diagnostic data. There is a little bit of O2 in the exhaust gas. Plus the converter creates O2 by disassociating NOx to O2 and N2. So if there is no O2 at the tail pipe, the three-way catalyst is working at peak efficiency."

"For a given speed and load the less ignition advance, the lower the peak combustion temperature, which lowers "engine out" NOx. At the same time it increases EGT, which will increase catalyst bed temperature."