Where the problem lies is the misconception that the downstream O2 sensor only monitors the health of the cat. What a lot of people don't understand is that the downstream sensor works in conjunction with the upstream sensor for adjusting the air fuel ratio through the PCM. Even though the MIL may not be triggered, eliminating the downstream sensor will cause the vehicle to operate in open loop mode. The result is decreased mpg and less than optimal engine performance.

 

I thought only 2005 Montes actually did this test?

Which is why ZZP has issues tuning for them and selling the downpipe for the 2005.