oxygen sensor readings

[Rich Mauruschat]

Grumpynomo made some very valid comments; it is surprisingly easy to be
misinformed by an apparently "weak" (low) reading which is actually due to
misfire. The effect is easily demonstrated if you can control the fuelling
by going richer-richer-richer and suddenly the high (>800mV) sensor output
shoots low when misfire occurs. During more normal running conditions this
effect can be a lot more subtle and easily misunderstood. The effect of only
one or two misfire cycles is very significant as a couple of cylinders' full
of fresh air are effectively chucked into the exhaust at an oxygen
concentration of 20 percent, the oxygen sensor is easily fast enough to
register this occurrence (when hot).
On another point raised elsewhere, my experience has shown that it IS
possible under steady-state conditions to show a mid-range reading of
stoich. (400mV- ish)  but under driving conditions the circumstances for
this to occur are very difficult to achieve and switching between levels is
more likely the norm. This statement is obviously (hopefully!) only true on
an open-loop fuelling system; on a closed loop system (for you lucky chaps
with mandatory catalysts <smile>) the signal will always switch between
levels due to the fuelling control loop. 
However the value of the oxygen sensor as an AFR indicator cannot be
disputed if you only want to know RICH - rich - weak - WEAK - HOLEINPISTONWEAK!!
good luck
Rich.