Rich mixture is indicated when the O2 content in the exhaust is extremely low compared to O2 in free air. NOx is created when combustion temperatures are very high. If high combustion temperatures are causing free oxygen to react with nitrogen then it will not be read by the O2 sensor. The 3.1 timing maps are generally very advanced at the load and speed where your car failed and require EGR to reduce NOx. Checking EGR is important.