On obdll emissions, what should the downstream o2 waveform look like with a good cat?

Question

on the post cat o2, should the voltage vary from about 250-600mv with a slow switch time, and under heavy load demand should you see rich condition for a moment(like the upstream) or should the mv stay lower? what does the ecm look for to set a po420(cat eff.)

Answer 1

How old a vehicle?
Does the vehicle have standard titania or zirconia O2s, or are they wide-band air/fuel monitors?
The strategy varies from manufacturer to manufacturer, and from year to year.
As a rule, the rear O2s should be relatively flat, and show a JUST slightly rich (low oxygen) condition. They really shouldn't be switching very much unless the vehicle is newer and the PCM is performing a cat efficiency test.
The rationale is that if the cat is functioning up to par, then the rear sensors shouldn't be following the front sensors. If it does, there isn't much catalyst change occuring in the cat.
The O2 measures oxygen. We don't want much oxygen in the exhaust stream. It becomes a pollutant when mixed with other exhaust components, and it also indicates an out-of-fuel-control condition.

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