Originally Posted by VEGASROBBI

Going off memory here but I think 09' was the year they added the rear converter's; got a dozen sitting on the floor of the shop. Converters are responsible to convert bad gasses into good gasses. NOx, CO, HC bad; O2, H²O, CO²(greenhouse).... not so bad. Basically converters do it by combining elements with oxygen. The elements that go into the engine through primary air and fuel come out in the same volume it's the makeup they come out that matters. In a proper running engine oxygen goes in at about 21% through primary air intake and exits the exhaust at 1.5% at stoich. Because oxygen is so important in the catalytic converter process early on we had aspirated converters that had oxygen plumbed to them for quicker warm ups and better efficiency. As engines got better in combustion efficiency the load on the converters has decreased. There are three main gases that converters target, NOx, HC and CO. HC is essentially raw fuel, CO is essentially partially burned fuel and NOx is nitrogen and oxygen melted together in various quantities. NO⁴ is one nitrogen atom which makes up about 79% of our atmosphere and four oxygen atoms. The problem with NO⁴ is it considered photochemical smog when it gets into the atmosphere it causes that brown haze which we call smog(smoke and fog). I grew up in LA in the 60's and 70's when smog was a big issue. Even though they had been leaning engines out since the 1950s smog kept getting worse. What they did not understand was leaning the motors out reduced hydrocarbons and carbon monoxide but it increased combustion chamber temperatures which increased NOx. NOx is formed under high heat about 2,500 degrees, that is when nitrogen and oxygen melt together from the free atmosphere. To make a short story long it was difficult to break up NOx because it required high temperatures to create NOx so equally it required high temperatures to break it down. EGR valves did a very good job of reducing NOx emissions by lowering combustion chamber temperature by resurculating exhaust gas which is inert back into the intake stream. Several converter configurations where tried to reduce NOx originally HC & Co converters were used to good effect. A third converter was added Upstream of the two-way converter because it received higher-temperature exhaust gases from combustion and that was considered the NOx converter. It's job was to break down NOx and the downstream converters job was to take care of CO's and HC's. Aspirated converters and heated converters with catalytic converter temperature sensors were tried in the 80s but they were complex and did not last long. Eventually three way converters that took care of all three gases were developed and worked very good. In some circumstances where higher emission standards required or a less efficient engine was in production or you were in California it was required to have a separate NOx converter to get the efficiency required. For some reason the JK added converters through its 3.8 L production, I don't know if it was pressure from the feds to make the 3.8 cleaner being an old design or Chrysler choose too in order to meet upcoming standards but they did change the converter configuration overtime.