dual lambda sensors]

[Dale Ulan]

> How does it work? What is gained by using the second lambda
> sensor? Isn't the oxygen that the first lambda sensor measures
> used up in the catalyst so there is no oxygen left for the
> second lambda sensor to measure?

What the O2 sensor measures isn't as straigtforward as you'd think.
The sensor measures rich/lean as everybody expects... rich = .8 volts,
lean = .1 volts (approx). The O2 sensor doesn't really respond to
oxygen content, since engine misfires don't cause just a LEAN indication...
If the engine is misfiring, a misfire rate of up to 2% gives an indication
of LEAN, but RICH if it the misfire rate is over 3%. 
...
The p-p voltage output
of the downstream sensor is used to determine catalyst efficiency. The
cat works by oxygen storage: NOx oxygen is used to get rid of HC and CO.
This alternating effect causes the downstream sensor to normally read
a constant (normally close to 100 mv), since the cat averages everything
out. If the cat looses efficiency, the output voltage starts to get
rich 'bumps' in the output.
If you have access to SAE papers, 920289 is a pretty good one on these
sensors.

-Dale