MSefi.com read only forum (Jan.15,2018)

I think I already know the answer to this.. But I thought I'd ask anyways.. Working on a car with a cat bolted directly to the turbo. So if I install an O2 bung downstream of the turbo, it'll be after a cat. The factory narrowband sensors are installed directly into the turbo, but a wideband would overheat in that location, wouldn't it? But after a cat, will it get anything near a correct reading?

Brian,

Regrdless of the heat, the only thing a O2 sensor after a catalytic converter will tell you is how effective the converter has been. This is because the converter's purpose is to alter the exhaust gas composition to be a clean as possible by converting the oxygen in the exhaust (and NOx) to CO2 and H2O.

That's why OEM's only use narrow band O2 sensors after converters - because all you can tell is if the converter did it's job or not (and that's all they need to know). There's more here: http://www.megamanual.com/ms2/tune.htm#catcon

Lance.

The readings after the catalytic converter would not be correct. The cat is designed to help complete the combustion of the unburned fuel in the exhaust. The output of the catalytic converter represents the combustion in the engine and the secondary combustion in the converter. Some cats (3 way) generate additional O2 by an additional reaction which is then used to help burn off the unburned fuel. They are scary good at cleaning up emissions in the range they operate at, but can clog up cause problems if run outside of those ranges. They like to operate at 14.7:1 AFR with a slight bit of alternating back and forth, which works well a narrow band sensor. A lot of the newer cars do have O2 sensors before and after the cat, these are used to help determine the catalytic converter's performance.

- Charles.